refactor(frontend): migrate Svelte stores from .ts to .svelte.ts runes

- Delete legacy .ts stores (auth, activity, assistantChat, datasetReview, environmentContext, health, sidebar, taskDrawer, translationRun)
- Create new .svelte.ts runes-based stores using  reactive primitives
- Migrate i18n: /i18n → /i18n/index.svelte.js
- Migrate toasts: /toasts → /toasts.svelte.js
- Update all component imports across 180+ files: components, pages, routes, lib
- Remove fromStore() wrappers — use store.value directly with Svelte 5 runes
- Update test mocks for new import paths
- Add @DEPRECATED annotation to legacy  alias

frontend/src/components/auth/ProtectedRoute.svelte

@@ -40,7 +40,7 @@
 
 <script lang="ts">
     import { onMount } from "svelte";
-    import { auth } from "../../lib/auth/store";
+    import { auth } from "../../lib/auth/store.svelte.js";
     import { api } from "../../lib/api";
     import { goto } from "$app/navigation";
     import { hasPermission } from "$lib/auth/permissions.js";