chore(frontend): real ESLint instead of a placeholder; fix hook order (FLAW-19)

eslint.config.js with the recommended JS/TypeScript rules and the React
hooks rules; npm run lint runs ESLint and is part of the image build.
Errors fail the build, the 24 exhaustive-deps findings stay warnings.

It found two real bugs: TaskDetailModal and ManageColumnsModal called
their hooks after 'if (!isOpen) return null', so the hook count changed
whenever the dialog opened or closed. Both are now a thin wrapper plus a
content component. Also removes unused imports and dead code (old task
list share handlers, an unused realtime status, an unused rollup).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/lists/SharedListsView.tsx

@@ -7,10 +7,9 @@ import {
   Radio,
   Sparkles,
   X,
-  Hash,
   ShoppingCart,
 } from 'lucide-react';
-import { ListItem, SharedList, ListShare, Team, User, WSMessage } from '../../lib/types';
+import { ListItem, SharedList, Team, User, WSMessage } from '../../lib/types';
 import { api, triggerHaptic, createRealtimeConnection, RealtimeTransport } from '../../lib/api';
 import { UnifiedShareModal } from '../../components/UnifiedShareModal';
 import { AvatarStack, AvatarPerson } from '../../components/AvatarStack';
@@ -502,7 +501,7 @@ export const SharedListsView: React.FC<SharedListsViewProps> = ({ onRefreshGloba
     if (!sourceItem) return;
 
     const targetCatNormalized = targetCategory.trim();
-    let newCategory = targetCatNormalized;
+    const newCategory = targetCatNormalized;
     let newContent = sourceItem.content;
 
     if (!targetCatNormalized) {