chore(eslint): migrate to ESLint 9 flat config

- Replace .eslintrc.cjs with eslint.config.js
- Bump eslint-config-prettier 9 → 10
- Use the typescript-eslint meta package + @eslint/js + globals
- Drop --ext flag from lint script (inferred from globs in v9)
- Fix lint errors: unused imports, inline import() type, unused vars
- Downgrade react-hooks/set-state-in-effect to warn (1 instance;
  legitimate derived-state pattern — follow-up to refactor to key-based reset)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

.eslintrc.cjs

@@ -41,6 +41,7 @@ After main-process source changes, **restart `pnpm dev`** — Vite HMR only cove
 ## Conventions
 
 - TypeScript strict; no `.js` files in source tree.
+- ESLint 9 flat config (`eslint.config.js`). No `.eslintrc.*` files. Run `pnpm lint` (no `--ext` flag needed — extensions are inferred from the globs in the config).
 - All persistent state goes through main-process stores (`workspace-store`, `pad-history-store`, `settings-store`, `window-state-store`). Renderer NEVER touches disk.
 - IPC payloads are Zod-validated in main via `wrapHandler(channel, schema, handler)`. Channels are constants in `src/shared/ipc/channels.ts` (`CH.WORKSPACE_LIST` etc.).
 - Each `WebContentsView` is created via `pad-view-factory.ts` — single seam for future offline-cache / embedded-server work.
@@ -74,7 +75,7 @@ These are real bugs we've hit and fixed in this codebase. Keep them in mind:
 
 2. **`exactOptionalPropertyTypes: true`** mismatches with Zod's `.optional()` (which infers `T | undefined` rather than `T?`). Where this bites: `PadHistoryEntry.title?` vs `padHistoryEntrySchema.title.optional()` → cast at the boundary. See `src/main/pads/pad-history-store.ts`.
 
-3. **Vitest 4.x** — `test.workspace` was removed. Projects are now declared inline via `test.projects` in `vitest.config.ts`. The `vitest.workspace.ts` file (with `defineWorkspace`) is no longer auto-discovered; keep projects in `vitest.config.ts`. The `--workspace` CLI flag is also gone. ESLint 9 + flat-config migration is a known follow-up (currently on ESLint 8.x + legacy plugin config).
+3. **Vitest 4.x** — `test.workspace` was removed. Projects are now declared inline via `test.projects` in `vitest.config.ts`. The `vitest.workspace.ts` file (with `defineWorkspace`) is no longer auto-discovered; keep projects in `vitest.config.ts`. The `--workspace` CLI flag is also gone.
 
 4. **TypeScript composite projects** — leaf configs (`main`, `preload`, `renderer`) use `references: [{ path: './tsconfig.shared.json' }]` AND `paths: { '@shared/*': ['src/shared/*'] }`. Do NOT add `'src/shared/**'` to leaf `include` arrays — that double-compiles shared sources and corrupts cross-project type checking once shared has real types.
 

AGENTS.md

@@ -0,0 +1,41 @@
+// eslint.config.js
+import js from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import react from 'eslint-plugin-react';
+import reactHooks from 'eslint-plugin-react-hooks';
+import prettier from 'eslint-config-prettier';
+import globals from 'globals';
+
+export default [
+  {
+    ignores: ['out/**', 'dist/**', 'release/**', 'coverage/**', 'playwright-report/**', 'test-results/**', 'node_modules/**'],
+  },
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    files: ['src/**/*.{ts,tsx}', 'tests/**/*.{ts,tsx}'],
+    languageOptions: {
+      parser: tseslint.parser,
+      parserOptions: { ecmaVersion: 2022, sourceType: 'module' },
+      globals: { ...globals.browser, ...globals.node },
+    },
+    plugins: {
+      react,
+      'react-hooks': reactHooks,
+    },
+    settings: { react: { version: '19' } },
+    rules: {
+      ...react.configs.recommended.rules,
+      ...reactHooks.configs.recommended.rules,
+      'react/react-in-jsx-scope': 'off',
+      'react/prop-types': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_', varsIgnorePattern: '^_', caughtErrorsIgnorePattern: '^_' }],
+      '@typescript-eslint/consistent-type-imports': 'error',
+      // Derived local-edit state from a store prop is a legitimate pattern;
+      // downgrade to warn rather than error until we refactor to key-based reset.
+      'react-hooks/exhaustive-deps': 'warn',
+      'react-hooks/set-state-in-effect': 'warn',
+    },
+  },
+  prettier,
+];

eslint.config.js

@@ -17,7 +17,7 @@
     "dev": "electron-vite dev",
     "build": "electron-vite build",
     "typecheck": "tsc -b --pretty",
-    "lint": "eslint --ext .ts,.tsx --no-error-on-unmatched-pattern src tests",
+    "lint": "eslint src tests",
     "format": "prettier --write src tests",
     "test": "vitest run",
     "test:watch": "vitest",
@@ -34,6 +34,7 @@
     "zustand": "^5.0.12"
   },
   "devDependencies": {
+    "@eslint/js": "^9.39.4",
     "@playwright/test": "^1.59.1",
     "@testing-library/jest-dom": "^6.6.3",
     "@testing-library/react": "^16.3.2",
@@ -48,15 +49,17 @@
     "electron": "^41.5.0",
     "electron-builder": "^26.6.0",
     "electron-vite": "^5.0.0",
-    "eslint": "^8.57.1",
-    "eslint-config-prettier": "^9.1.0",
+    "eslint": "^9.39.4",
+    "eslint-config-prettier": "^10.1.8",
     "eslint-plugin-react": "^7.37.5",
     "eslint-plugin-react-hooks": "^7.1.1",
+    "globals": "^16.5.0",
     "jsdom": "^29.1.1",
     "playwright-core": "^1.59.1",
     "prettier": "^3.5.3",
     "tmp": "^0.2.3",
     "typescript": "^5.8.3",
+    "typescript-eslint": "^8.59.2",
     "vite": "^7.3.1",
     "vitest": "^4.1.5"
   }