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/GitOperationsPanel.svelte

@@ -2,7 +2,7 @@
 <!-- @BRIEF Git server operations panel: pull, push, and deploy actions. -->
 <!-- @LAYER UI -->
 <script lang="ts">
-  import { t } from "$lib/i18n";
+  import { t } from "$lib/i18n/index.svelte.js";
   import { Button } from "$lib/ui";
 
   let {