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/git/BranchSelector.svelte

@@ -16,7 +16,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import { BranchModel } from '$lib/models/BranchModel.svelte.ts';
-  import { t } from '$lib/i18n';
+  import { t } from '$lib/i18n/index.svelte.js';
   import { Button, Select, Input } from '$lib/ui';
 
   let {