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/help/HelpView.tsx

@@ -6,7 +6,6 @@ import {
   CheckSquare,
   Keyboard,
   Pin,
-  FolderTree,
   Sparkles,
   Command,
   Users,
@@ -19,14 +18,10 @@ import {
   Lock,
   ArrowLeft,
   X,
-  Smartphone,
-  RotateCcw,
   Heading1,
   ListTodo,
   Code,
   Download,
-  Flame,
-  Check,
 } from 'lucide-react';
 import { AboutModal } from '../../components/AboutModal';