fix: give SensitiveInput a unique default id (WCAG 1.3.1, 4.1.2) (#27488)

On latest `dev`, `SensitiveInput` defaults to `export let id = 'password-input'`. The id is used both for the input itself and as the `for` target of the screen reader label rendered just above it.

There are 80 `<SensitiveInput>` usages in `src/` and only 4 pass an explicit id, so the remaining 76 all render `id="password-input"` together with `<label for="password-input">`. These collide on the same page in completely ordinary configurations: `admin/Settings/Audio.svelte` renders 4 at once with `STT_ENGINE === 'openai'` and 4 more with `TTS_ENGINE === 'openai'`, `admin/Settings/Documents.svelte` has 11, and `admin/Settings/WebSearch.svelte` has 33.

`for` resolves to the first matching element, so every label after the first points at the wrong input. In practice a screen reader user tabbing to the OpenAI TTS API key field hears the label belonging to the STT key field from a different section, and every one of those fields announces the same name. Browser password managers and any `getElementById` lookup collapse onto the first element the same way.

Breaks WCAG 1.3.1 Info and Relationships (Level A), because the programmatic label/field relationship is wrong, and 4.1.2 Name, Role, Value (Level A), because the fields do not expose their correct accessible name.

Fix: default the id to a per instance unique value. A Svelte prop default is evaluated per component instance, so each `SensitiveInput` gets its own stable id, and the 4 call sites that pass an explicit id are unaffected. `uuid` is already a direct dependency and `import { v4 as uuidv4 } from 'uuid'` is the existing pattern in the codebase, including `common/Collapsible.svelte`, which already generates a DOM id this way.

Note for self hosted setups: a `#password-input` selector in `static/custom.css` would stop matching. That selector already matched up to 8 elements at once on the Audio settings page, so it was never a reliable hook.

Severity: Serious. Every API key field in Admin Settings is mislabelled for assistive technology.

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src/lib/components/common/SensitiveInput.svelte

@@ -1,7 +1,8 @@
 <script lang="ts">
 	const i18n = getContext('i18n');
 	import { getContext } from 'svelte';
-	export let id = 'password-input';
+	import { v4 as uuidv4 } from 'uuid';
+	export let id = `password-input-${uuidv4()}`;
 	export let value: string = '';
 	export let placeholder = '';
 	export let type = 'text';