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/RepositoryDashboardGrid.svelte

@@ -14,11 +14,11 @@
   // [SECTION: IMPORTS]
   import { untrack } from "svelte";
   import type { DashboardMetadata } from "../types/dashboard";
-  import { t } from "../lib/i18n";
+  import { t } from '$lib/i18n/index.svelte.js';
   import { Button, Input } from "../lib/ui";
   import GitManager from "./git/GitManager.svelte";
   import { gitService } from "../services/gitService";
-  import { addToast } from "../lib/toasts";
+  import { addToast } from "$lib/toasts.svelte.js";
   // [/SECTION]
 
   // [SECTION: PROPS]