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/docSync/__tests__/diff.test.ts

@@ -49,7 +49,7 @@ describe('diffFlat', () => {
       return Math.floor((seed / 2147483648) * n);
     };
     let current: DocumentBlock[] = [B('x0', 0)];
-    let d = build(current);
+    const d = build(current);
     for (let step = 0; step < 300; step++) {
       const next = current.map((b) => ({ ...b }));
       const i = rnd(next.length);