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/lib/api.ts

@@ -17,7 +17,6 @@ import {
   SharedFolder,
   FolderShare,
   Space,
-  SpaceMember,
   SpaceShare,
   SystemSettings,
   SharingMode,
@@ -1544,7 +1543,7 @@ export async function connectWebSocket(): Promise<void> {
       }
     };
 
-    ws.onerror = (e) => {
+    ws.onerror = () => {
       clearTimeout(wsConnectionWatchdog);
       switchToSSE('WebSocket Fehler (Handshake/Verbindung verweigert)');
     };