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/tests/maintenance.test.ts

@@ -47,12 +47,12 @@ function resetMockStore() {
 }
 
 // Mock toasts
-vi.mock('$lib/toasts.js', () => ({
+vi.mock('$lib/toasts.svelte.js', () => ({
     addToast: vi.fn()
 }));
 
 // Mock i18n — $t is a derived store, needs subscribe method for Svelte $ prefix
-vi.mock('$lib/i18n', () => {
+vi.mock('$lib/i18n/index.svelte.js', () => {
     const subscribers = new Set();
     const tData = {
         maintenance: {
@@ -83,6 +83,7 @@ vi.mock('$lib/i18n', () => {
             }
         },
         _: (key) => key,
+        getT: () => ({}),
     };
 });