feat: use CodeMirror for always-editable code file preview

- Add FileCodeEditor.svelte: CodeMirror wrapper with auto language
  detection, dark mode, Ctrl+S save, reactive to value/filePath changes
- Replace Shiki read-only highlighting + textarea editing with
  always-editable CodeMirror for code files in FileNav preview
- Show persistent Save button for code files in toolbar
- Non-code text files keep existing Edit/Save/Cancel textarea flow
- SVG retains Shiki highlighting for visual preview mode

src/lib/components/chat/FileNav.svelte

@@ -659,7 +659,7 @@
 					</button>
 				</Tooltip>
 			{/if}
-			{#if (isMarkdown || isCsv || isHtml || isCode || isJson || isSvg || isNotebook) && fileContent !== null && !editing}
+			{#if (isMarkdown || isCsv || isHtml || isJson || isSvg || isNotebook) && fileContent !== null && !editing}
 				<Tooltip content={showRaw ? $i18n.t('Preview') : $i18n.t('Source')}>
 					<button
 						class="shrink-0 p-1 rounded hover:bg-gray-100 dark:hover:bg-gray-800 transition text-gray-400 dark:text-gray-500 hover:text-gray-600 dark:hover:text-gray-400"
@@ -709,7 +709,24 @@
 				</Tooltip>
 			{/if}
 			{#if isTextFile}
-				{#if editing}
+				{#if isCode}
+					<Tooltip content={$i18n.t('Save')}>
+						<button
+							class="shrink-0 p-1 rounded hover:bg-gray-100 dark:hover:bg-gray-800 transition text-gray-400 dark:text-gray-500 hover:text-gray-600 dark:hover:text-gray-400"
+							on:click={() => filePreviewRef?.saveCodeFile()}
+							disabled={saving}
+							aria-label={$i18n.t('Save')}
+						>
+							{#if saving}
+								<Spinner className="size-3.5" />
+							{:else}
+								<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor" class="size-3.5">
+									<path fill-rule="evenodd" d="M16.704 4.153a.75.75 0 0 1 .143 1.052l-8 10.5a.75.75 0 0 1-1.127.075l-4.5-4.5a.75.75 0 0 1 1.06-1.06l3.894 3.893 7.48-9.817a.75.75 0 0 1 1.05-.143Z" clip-rule="evenodd" />
+								</svg>
+							{/if}
+						</button>
+					</Tooltip>
+				{:else if editing}
 					<Tooltip content={$i18n.t('Cancel')}>
 						<button
 							class="shrink-0 p-1 rounded hover:bg-gray-100 dark:hover:bg-gray-800 transition text-gray-400 dark:text-gray-500 hover:text-gray-600 dark:hover:text-gray-400"

src/lib/components/chat/FileNav/FileCodeEditor.svelte

@@ -0,0 +1,139 @@
+<script lang="ts">
+	import '$lib/utils/codemirror';
+	import { basicSetup, EditorView } from 'codemirror';
+	import { keymap } from '@codemirror/view';
+	import { Compartment, EditorState } from '@codemirror/state';
+	import { indentWithTab } from '@codemirror/commands';
+	import { indentUnit, LanguageDescription } from '@codemirror/language';
+	import { languages } from '@codemirror/language-data';
+	import { oneDark } from '@codemirror/theme-one-dark';
+	import { onMount, onDestroy } from 'svelte';
+
+	export let value = '';
+	export let filePath: string | null = null;
+	export let onSave: ((content: string) => Promise<void>) | null = null;
+
+	let container: HTMLDivElement;
+	let editor: EditorView | null = null;
+	let editorTheme = new Compartment();
+	let editorLanguage = new Compartment();
+	let internalValue = '';
+
+	/** Return the current editor content */
+	export const getValue = (): string => {
+		return editor?.state.doc.toString() ?? value;
+	};
+
+	/** Replace editor content */
+	export const setValue = (newValue: string) => {
+		if (!editor) return;
+		internalValue = newValue;
+		editor.dispatch({
+			changes: { from: 0, to: editor.state.doc.length, insert: newValue }
+		});
+	};
+
+	export const focus = () => {
+		editor?.focus();
+	};
+
+	const detectLanguage = async (path: string | null) => {
+		if (!path) return;
+		const match = LanguageDescription.matchFilename(languages, path);
+		if (match) {
+			const lang = await match.load();
+			if (lang && editor) {
+				editor.dispatch({ effects: editorLanguage.reconfigure(lang) });
+			}
+		}
+	};
+
+	// React to external value changes (e.g. switching files)
+	$: if (editor && value !== internalValue) {
+		internalValue = value;
+		editor.dispatch({
+			changes: { from: 0, to: editor.state.doc.length, insert: value }
+		});
+	}
+
+	// React to filePath changes for language detection
+	$: if (editor && filePath) {
+		detectLanguage(filePath);
+	}
+
+	onMount(() => {
+		const isDark = document.documentElement.classList.contains('dark');
+		internalValue = value;
+
+		const extensions = [
+			basicSetup,
+			keymap.of([
+				indentWithTab,
+				{
+					key: 'Mod-s',
+					run: () => {
+						if (onSave) {
+							onSave(editor?.state.doc.toString() ?? '');
+						}
+						return true;
+					}
+				}
+			]),
+			indentUnit.of('    '),
+			EditorView.updateListener.of((e) => {
+				if (e.docChanged) {
+					internalValue = e.state.doc.toString();
+					value = internalValue;
+				}
+			}),
+			editorTheme.of(isDark ? oneDark : []),
+			editorLanguage.of([]),
+			EditorView.theme({
+				'&': { fontSize: '0.75rem', height: '100%' },
+				'.cm-content': {
+					padding: '0.5rem 0',
+					fontFamily: 'ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace'
+				},
+				'.cm-scroller': { overflow: 'auto' },
+				'.cm-focused': { outline: 'none' }
+			})
+		];
+
+		editor = new EditorView({
+			state: EditorState.create({ doc: value, extensions }),
+			parent: container
+		});
+
+		detectLanguage(filePath);
+
+		// Watch dark mode
+		const observer = new MutationObserver(() => {
+			const dark = document.documentElement.classList.contains('dark');
+			editor?.dispatch({ effects: editorTheme.reconfigure(dark ? oneDark : []) });
+		});
+		observer.observe(document.documentElement, { attributes: true, attributeFilter: ['class'] });
+
+		return () => {
+			observer.disconnect();
+			editor?.destroy();
+			editor = null;
+		};
+	});
+
+	onDestroy(() => {
+		editor?.destroy();
+		editor = null;
+	});
+</script>
+
+<div bind:this={container} class="file-code-editor" />
+
+<style>
+	.file-code-editor {
+		width: 100%;
+		height: 100%;
+	}
+	.file-code-editor :global(.cm-editor) {
+		height: 100%;
+	}
+</style>

src/lib/components/chat/FileNav/FilePreview.svelte

@@ -4,15 +4,17 @@
 	import { marked } from 'marked';
 	import DOMPurify from 'dompurify';
 	import { settings } from '$lib/stores';
-	import { isCodeFile, highlightCode } from '$lib/utils/codeHighlight';
+	import { isCodeFile } from '$lib/utils/codeHighlight';
 	import { initMermaid, renderMermaidDiagram } from '$lib/utils';
 	import Spinner from '../../common/Spinner.svelte';
 	import PDFViewer from '../../common/PDFViewer.svelte';
 	import JsonTreeView from './JsonTreeView.svelte';
 	import NotebookView from './NotebookView.svelte';
 	import SqliteView from './SqliteView.svelte';
+	import FileCodeEditor from './FileCodeEditor.svelte';
 
 	let pdfViewerRef: PDFViewer;
+	let fileCodeEditorRef: FileCodeEditor;
 
 	const i18n = getContext('i18n');
 
@@ -76,6 +78,15 @@
 		editContent = '';
 	};
 
+	/** Save code file directly from CodeMirror */
+	export const saveCodeFile = async () => {
+		if (!onSave) return;
+		saving = true;
+		const content = fileCodeEditorRef?.getValue() ?? '';
+		await onSave(content);
+		saving = false;
+	};
+
 	$: isTextFile = fileContent !== null && fileImageUrl === null && filePdfData === null;
 
 	const MD_EXTS = new Set(['md', 'markdown', 'mdx']);
@@ -175,24 +186,21 @@
 	$: csvHeader = csvRows.length > 0 ? csvRows[0] : [];
 	$: csvBody = csvRows.length > 1 ? csvRows.slice(1) : [];
 
-	// ── Shiki code highlighting ─────────────────────────────────────────
+	// ── Shiki code highlighting (SVG only) ──────────────────────────────
 	let highlightedHtml: string | null = null;
-	let highlightingFile: string | null = null; // track which file we're highlighting
+	let highlightingFile: string | null = null;
 
-	$: if ((isCode || isSvg) && fileContent !== null && selectedFile) {
+	$: if (isSvg && fileContent !== null && selectedFile) {
 		const currentFile = selectedFile;
 		highlightingFile = currentFile;
-		const lang = isSvg ? 'xml' : undefined;
-		(lang
-			? import('shiki').then(({ codeToHtml }) =>
-					codeToHtml(fileContent!, {
-						lang: 'xml',
-						themes: { light: 'github-light', dark: 'github-dark' },
-						defaultColor: 'light'
-					})
-				)
-			: highlightCode(fileContent!, selectedFile!)
-		)
+		import('shiki')
+			.then(({ codeToHtml }) =>
+				codeToHtml(fileContent!, {
+					lang: 'xml',
+					themes: { light: 'github-light', dark: 'github-dark' },
+					defaultColor: 'light'
+				})
+			)
 			.then((html) => {
 				if (highlightingFile === currentFile) highlightedHtml = html;
 			})
@@ -428,7 +436,16 @@
 			<div class="svg-preview w-full h-full flex items-center justify-center overflow-auto p-3">
 				{@html DOMPurify.sanitize(fileContent, { USE_PROFILES: { svg: true, svgFilters: true }, ADD_TAGS: ['use'] })}
 			</div>
-		{:else if (isCode || isSvg) && highlightedHtml && !showRaw}
+		{:else if isCode && !showRaw}
+			<div class="h-full">
+				<FileCodeEditor
+					bind:this={fileCodeEditorRef}
+					value={fileContent ?? ''}
+					filePath={selectedFile}
+					{onSave}
+				/>
+			</div>
+		{:else if isSvg && highlightedHtml && !showRaw}
 			<div class="shiki-preview overflow-auto h-full text-xs">
 				{@html highlightedHtml}
 			</div>