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/package.json

@@ -8,7 +8,7 @@
     "dev": "vite",
     "build": "vite build",
     "preview": "vite preview",
-    "lint": "echo 'Linting passed'",
+    "lint": "eslint .",
     "test": "vitest",
     "test:run": "vitest run"
   },
@@ -22,17 +22,22 @@
     "react-dom": "^18.3.1"
   },
   "devDependencies": {
+    "@eslint/js": "9.17.0",
     "@testing-library/dom": "^10.4.2",
     "@testing-library/react": "^16.3.3",
     "@types/react": "^18.3.12",
     "@types/react-dom": "^18.3.1",
     "@vitejs/plugin-react": "^4.3.4",
     "autoprefixer": "^10.4.20",
+    "eslint": "9.17.0",
+    "eslint-plugin-react-hooks": "5.1.0",
     "fake-indexeddb": "^6.2.6",
+    "globals": "15.14.0",
     "jsdom": "^25.0.1",
     "postcss": "^8.4.49",
     "tailwindcss": "^3.4.17",
     "typescript": "^5.6.3",
+    "typescript-eslint": "8.18.2",
     "vite": "^5.4.11",
     "vitest": "^2.1.9"
   }