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/App.tsx

@@ -470,7 +470,7 @@ export const App: React.FC = () => {
       .getThemes()
       .then((themes) => handleCustomThemesChange(themes))
       .catch(() => {});
-    // eslint-disable-next-line react-hooks/exhaustive-deps
+     
   }, [user?.id]);
 
   const refreshAll = useCallback(async () => {