Add lint, typecheck, and html-validate guardrails

Tooling:
- ESLint 9 flat config (eslint.config.mjs) with three scopes:
  - browser vanilla JS for js/ (no modules, window globals)
  - Node ESM for scripts/
  - typescript-eslint for tests/ and *.ts
- Stylelint with a deliberately minimal "bug-only" ruleset
  (block-no-empty, color-no-invalid-hex, function-no-unknown,
  property-no-unknown, …) — no nags about compact handwritten CSS
- html-validate against index.html (DOCTYPE, accessible names,
  non-redundant ARIA roles, valid landmark usage)
- TypeScript --noEmit strict on playwright.config.ts + tests/*.ts

npm scripts:
- lint        — parallel run of lint:js, lint:css, lint:html (~3s)
- lint:js{,:fix}, lint:css{,:fix}, lint:html
- typecheck   — tsc --noEmit
- check       — lint + typecheck + test in parallel (CI entry point)

Fixes uncovered by the new checks:
- js/app.js: drop unused `activeTimers`, drop unused `t` lookup in
  refreshTreeRow, switch `activeTags` let→const, replace empty
  `catch(_)` blocks with parameter-less catch + intent comment
- js/data.js: drop unused index arg in renderExperience
- index.html: uppercase DOCTYPE, drop redundant role="banner" /
  role="contentinfo" on <header>/<footer>, add aria-labels on icon-only
  Stop / Reset buttons, add role="group" to the tag bar so its
  aria-label is valid, add explicit type="button" everywhere
- tests/portfolio.spec.ts: landmark test now uses getByRole() rather
  than the explicit [role="banner"] attribute selector

Housekeeping:
- .gitignore picks up .eslintcache / .stylelintcache / *.tsbuildinfo
- README documents the lint + check toolchain

Co-authored-by: Cursor <cursoragent@cursor.com>

eslint.config.mjs

@@ -0,0 +1,89 @@
+// Flat config — ESLint 9. Three lint targets:
+//   1. Browser vanilla JS in `js/`           (no build, no modules)
+//   2. Node ES modules in `scripts/`         (utility tasks)
+//   3. TypeScript test + config files        (@playwright/test)
+import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+
+export default [
+  {
+    ignores: [
+      "node_modules/",
+      "playwright-report/",
+      "test-results/",
+      "assets/",
+      "dist/",
+      ".cache/",
+    ],
+  },
+
+  // ----- Browser vanilla JS (js/app.js, js/data.js) -----
+  {
+    files: ["js/**/*.js"],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: "script",
+      globals: {
+        ...globals.browser,
+        // `data.js` exposes window.PORTFOLIO and a set of `renderXxx`
+        // top-level functions referenced from inline `render:` properties.
+        // They are looked up by reference inside data.js, not by name from
+        // other files, so we don't need to pre-declare each renderer.
+        PORTFOLIO: "readonly",
+      },
+    },
+    rules: {
+      ...js.configs.recommended.rules,
+      eqeqeq: ["error", "always"],
+      "no-var": "error",
+      "prefer-const": "warn",
+      "no-console": ["warn", { allow: ["warn", "error"] }],
+      // _ / _esc are intentional helper / placeholder names.
+      "no-unused-vars": [
+        "warn",
+        {
+          argsIgnorePattern: "^_",
+          varsIgnorePattern: "^_",
+          caughtErrorsIgnorePattern: "^_",
+        },
+      ],
+    },
+  },
+
+  // ----- Node ES modules (scripts/*.mjs) -----
+  {
+    files: ["scripts/**/*.{js,mjs}"],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: "module",
+      globals: { ...globals.node },
+    },
+    rules: {
+      ...js.configs.recommended.rules,
+      eqeqeq: ["error", "always"],
+      "no-var": "error",
+      "prefer-const": "warn",
+      "no-console": "off",
+    },
+  },
+
+  // ----- TypeScript (tests/, playwright.config.ts) -----
+  ...tseslint.configs.recommended.map((cfg) => ({
+    ...cfg,
+    files: ["tests/**/*.ts", "*.ts"],
+  })),
+  {
+    files: ["tests/**/*.ts", "*.ts"],
+    languageOptions: {
+      globals: { ...globals.node },
+    },
+    rules: {
+      "@typescript-eslint/no-unused-vars": [
+        "warn",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
+      ],
+      "@typescript-eslint/no-explicit-any": "off",
+    },
+  },
+];