fix(ui): empty-string i18n params rendered the raw {placeholder} (#123474)

t() interpolation used || so a provided empty-string param fell through to
the visible {placeholder} fallback meant for missing params — e.g. the
devices page rendered 'Bound to {node}' whenever an agent binding was the
empty string. ?? keeps the missing-param debugging aid and renders provided
empties as empty.

ui/src/i18n/lib/translate.ts

@@ -264,7 +264,9 @@ class I18nManager {
     }
 
     if (params) {
-      return value.replace(/\{(\w+)\}/g, (_, k) => params[k] || `{${k}}`);
+      // ?? not ||: an empty-string param is a provided value (render empty),
+      // while a missing param keeps the visible {placeholder} for debugging.
+      return value.replace(/\{(\w+)\}/g, (_, k) => params[k] ?? `{${k}}`);
     }
 
     return value;

ui/src/i18n/test/translate.test.ts

@@ -111,6 +111,14 @@ describe("i18n", () => {
     );
   });
 
+  it("renders a provided empty-string param as empty, not the raw placeholder", () => {
+    expect(translate.t("connection.help.copyCommandAria", { command: "" })).toBe("Copy command: ");
+  });
+
+  it("keeps the visible placeholder when the param is missing", () => {
+    expect(translate.t("connection.help.copyCommandAria", {})).toBe("Copy command: {command}");
+  });
+
   it("should fallback to English if key is missing in another locale", async () => {
     translate.i18n.registerTranslation("zh-CN", { common: {} } as never);
     await translate.i18n.setLocale("zh-CN");