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/eslint.config.js

@@ -0,0 +1,30 @@
+// Lint rules for the frontend (FLAW-19): real mistakes are errors, style is left to the code.
+import js from '@eslint/js';
+import globals from 'globals';
+import reactHooks from 'eslint-plugin-react-hooks';
+import tseslint from 'typescript-eslint';
+
+export default tseslint.config(
+  { ignores: ['dist', 'node_modules', 'coverage'] },
+  {
+    files: ['src/**/*.{ts,tsx}'],
+    extends: [js.configs.recommended, ...tseslint.configs.recommended],
+    languageOptions: {
+      ecmaVersion: 2022,
+      globals: globals.browser,
+    },
+    plugins: { 'react-hooks': reactHooks },
+    rules: {
+      ...reactHooks.configs.recommended.rules,
+      // Error payloads and browser APIs without types are handled as any on purpose
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_', varsIgnorePattern: '^_', caughtErrors: 'none' }],
+      'no-empty': ['error', { allowEmptyCatch: true }],
+    },
+  },
+  {
+    files: ['public/sw.js'],
+    languageOptions: { globals: globals.serviceworker },
+    extends: [js.configs.recommended],
+  },
+);