fix: let Select announce its selected value and open state (WCAG 2.5.3, 4.1.2) (#27492)

On latest `dev`, the `Select` trigger sets `aria-label={placeholder}`. In the accessible name computation `aria-label` is evaluated before the element's contents, so on a button that renders visible text it **replaces** that text instead of adding to it.

The trigger's content is `selectedLabel`, which resolves to the selected item's label and only falls back to `placeholder` when nothing is selected. So a control visually reading "Week" is exposed to assistive technology as "Select view", permanently, no matter what is selected. The dropdown items expose no `aria-selected` either, the current one is marked with a check icon only, so there is no path by which a screen reader user can find out what the control is set to.

Breaks WCAG 2.5.3 Label in Name (Level A), because the accessible name does not contain the visible label, so voice control cannot target the control by what it says on screen. Also 4.1.2 Name, Role, Value (Level A), because the value is never exposed.

Fix: drop the overriding `aria-label` so the name is computed from the visible text, and expose `aria-expanded` so the open state is conveyed. All 7 call sites plus the `DropdownOptions` wrapper override `slot="trigger"` and every one of them renders `selectedLabel` (or `placeholder`) as text, so no trigger is left unnamed. The two call sites that pass no `placeholder` were already emitting an empty `aria-label`, which is skipped by the name computation, so they are unaffected.

`aria-haspopup` is deliberately not added: the popup is a plain `DropdownMenu` of buttons with no `listbox` role, so claiming one would misdescribe it.

`placeholder` is still used, it drives the `selectedLabel` fallback and `TagSelector` renders it directly.

Severity: Serious. Affects every custom select in Admin Settings, Workspace and the calendar and automations pages.

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src/lib/components/common/Select.svelte

@@ -114,8 +114,8 @@
 <button
 	bind:this={triggerEl}
 	class={triggerClass}
-	aria-label={placeholder}
 	type="button"
+	aria-expanded={open}
 	on:click={toggleOpen}
 >
 	<slot name="trigger" {selectedLabel} {open}>