Add ESLint + pre-commit hook and fix the issues it surfaced

.husky/pre-commit

@@ -0,0 +1,4 @@
+# Format + lint the staged files, then run the full verify gate.
+# At this project's size all three finish in ~2s; if the suite grows slow,
+# move typecheck/test to a pre-push hook and leave lint-staged here.
+npx lint-staged && npm run typecheck && npm test

eslint.config.js

@@ -0,0 +1,54 @@
+import js from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import reactHooks from 'eslint-plugin-react-hooks';
+import reactRefresh from 'eslint-plugin-react-refresh';
+import globals from 'globals';
+
+export default tseslint.config(
+  // Ignore build output and generated artifacts.
+  { ignores: ['dist', 'dev-dist', 'coverage'] },
+
+  js.configs.recommended,
+
+  // Type-aware linting for the TypeScript sources only.
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [...tseslint.configs.recommendedTypeChecked],
+    languageOptions: {
+      ecmaVersion: 2022,
+      globals: { ...globals.browser, __APP_VERSION__: 'readonly' },
+      parserOptions: {
+        // Type-aware linting: powers no-floating-promises, no-misused-promises, etc.
+        projectService: true,
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    plugins: {
+      'react-hooks': reactHooks,
+      'react-refresh': reactRefresh,
+    },
+    rules: {
+      ...reactHooks.configs.recommended.rules,
+      'react-refresh/only-export-components': ['warn', { allowConstantExport: true }],
+      // Allow intentionally-unused args/vars when prefixed with _ (matches tsconfig intent).
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+    },
+  },
+
+  // Tests exercise looser patterns and run in Node.
+  {
+    files: ['**/*.test.{ts,tsx}'],
+    languageOptions: { globals: { ...globals.node } },
+  },
+
+  // Plain JS config files (this file, etc.) are not part of the TS project —
+  // run them through the untyped ruleset only.
+  {
+    files: ['**/*.js'],
+    extends: [tseslint.configs.disableTypeChecked],
+    languageOptions: { globals: { ...globals.node } },
+  },
+);

package.json

@@ -11,7 +11,17 @@
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
-    "format": "prettier --write ."
+    "lint": "eslint .",
+    "lint:fix": "eslint . --fix",
+    "format": "prettier --write .",
+    "prepare": "husky"
+  },
+  "lint-staged": {
+    "*.{ts,tsx}": [
+      "eslint --fix",
+      "prettier --write"
+    ],
+    "*.{json,css,md}": "prettier --write"
   },
   "dependencies": {
     "@fontsource/ibm-plex-mono": "^5.2.7",
@@ -25,12 +35,20 @@
     "zustand": "^5.0.14"
   },
   "devDependencies": {
+    "@eslint/js": "^10.0.1",
     "@types/react": "^19.2.16",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^5.2.0",
+    "eslint": "^10.4.1",
+    "eslint-plugin-react-hooks": "^7.1.1",
+    "eslint-plugin-react-refresh": "^0.5.2",
+    "globals": "^17.6.0",
     "happy-dom": "^20.0.0",
+    "husky": "^9.1.7",
+    "lint-staged": "^17.0.7",
     "prettier": "^3.6.2",
     "typescript": "^5.9.2",
+    "typescript-eslint": "^8.60.1",
     "vite": "^7.1.0",
     "vite-plugin-pwa": "^1.0.3",
     "vitest": "^3.2.4"

src/app/components/LivePreview.tsx

@@ -36,7 +36,10 @@ export function LivePreview() {
     if (!node) return;
     const text = draftText.trim();
 
-    const timer = setTimeout(async () => {
+    // The debounced body is async; wrap in a void IIFE so the timer callback
+    // returns void (it handles its own errors internally — nothing awaits it).
+    const timer = setTimeout(() => {
+      void (async () => {
         // Empty/blank is not an error — clean, empty pane (spec §04).
         if (text === '') {
           handleRef.current?.destroy();
@@ -80,6 +83,7 @@ export function LivePreview() {
             );
           }
         }
+      })();
     }, RENDER_DEBOUNCE_MS);
 
     return () => clearTimeout(timer);

src/app/infrastructure/db.ts

@@ -56,8 +56,9 @@ function tx<T>(
         const transaction = db.transaction(store, mode);
         const request = run(transaction.objectStore(store));
         transaction.oncomplete = () => resolve(request.result);
-        transaction.onerror = () => reject(transaction.error);
+        const fail = () => reject(transaction.error ?? new Error('IndexedDB transaction failed'));
-        transaction.onabort = () => reject(transaction.error);
+        transaction.onerror = fail;
+        transaction.onabort = fail;
       }),
   );
 }
@@ -69,10 +70,10 @@ export const getAll = <T>(store: string): Promise<T[]> =>
   tx<T[]>(store, 'readonly', (s) => s.getAll() as IDBRequest<T[]>);
 
 export const put = <T>(store: string, value: T): Promise<IDBValidKey> =>
-  tx<IDBValidKey>(store, 'readwrite', (s) => s.put(value as unknown as Record<string, unknown>));
+  tx<IDBValidKey>(store, 'readwrite', (s) => s.put(value));
 
 export const del = (store: string, key: IDBValidKey): Promise<undefined> =>
-  tx<undefined>(store, 'readwrite', (s) => s.delete(key) as IDBRequest<undefined>);
+  tx<undefined>(store, 'readwrite', (s) => s.delete(key));
 
 /** Test-only: forget the memoized connection so a fresh `openDB` reopens. */
 export function _resetDbForTests(): void {

src/app/infrastructure/settings-store.test.ts

@@ -71,7 +71,11 @@ describe('settings-store · ui.theme', () => {
 
       saveUiTheme('dark');
 
-      const stored = JSON.parse(localStorage.getItem(KEY)!);
+      const stored = JSON.parse(localStorage.getItem(KEY)!) as {
+        version: number;
+        ui: { theme: string; previewFitMode: string };
+        editor: { fontSize: number };
+      };
       expect(stored.ui.theme).toBe('dark');
       // Everything else survives — nothing clobbered.
       expect(stored.version).toBe(1);

src/app/infrastructure/settings-store.ts

@@ -40,7 +40,7 @@ function readRaw(): StoredSettings {
   try {
     const raw = localStorage.getItem(KEY);
     if (!raw) return {};
-    const parsed = JSON.parse(raw);
+    const parsed: unknown = JSON.parse(raw);
     return parsed && typeof parsed === 'object' ? (parsed as StoredSettings) : {};
   } catch (err) {
     console.warn('[settings] failed to read, using defaults', err);

src/core/format-detection.ts

@@ -19,9 +19,7 @@ export interface FormatDetection {
 
 function isTopology(value: unknown): boolean {
   return (
-    typeof value === 'object' &&
+    typeof value === 'object' && value !== null && (value as { type?: unknown }).type === 'Topology'
-    value !== null &&
-    (value as { type?: unknown }).type === 'Topology'
   );
 }
 
@@ -32,7 +30,7 @@ export function detectFormat(raw: string): FormatDetection {
 
   // 1. Try JSON first — highest confidence signal.
   try {
-    const parsed = JSON.parse(text);
+    const parsed: unknown = JSON.parse(text);
     return {
       format: isTopology(parsed) ? 'topojson' : 'json',
       confidence: 'high',

vite.config.ts

@@ -4,7 +4,9 @@ import { VitePWA } from 'vite-plugin-pwa';
 import { readFileSync } from 'node:fs';
 import { fileURLToPath } from 'node:url';
 
-const pkg = JSON.parse(readFileSync(new URL('./package.json', import.meta.url), 'utf-8'));
+const pkg = JSON.parse(readFileSync(new URL('./package.json', import.meta.url), 'utf-8')) as {
+  version: string;
+};
 
 export default defineConfig({
   resolve: {