build(frontend): add ESLint flat config as a gate lane

ESLint 9 flat config, pinned. Narrow by design: the load-bearing rule is
react-hooks (rules-of-hooks + exhaustive-deps) — 33 disable comments existed for a
rule nothing ran. tsc already owns unused vars / undefined / types, so those are
turned off here to report each finding once. no-explicit-any is off: all 19 live in
the api.ts god-module that R7 rewrites, and it flips the rule back on then.

reportUnusedDisableDirectives is an error, which immediately caught two inert
`eslint-disable` comments (App.tsx, AddToPlaylist.tsx) whose effects have stable
deps — removed. no-unused-expressions allows the side-effect ternary idiom the
codebase already uses (`v.paused ? v.play() : v.pause()`).

Errors: 0. Remaining 38 are warnings (28 react-refresh DX, 10 exhaustive-deps) —
pre-existing, visible in the lint output, tracked to their epics.

frontend/package.json

@@ -11,6 +11,9 @@
     "build": "npm run typecheck:app && npm run typecheck:node && vite build",
     "preview": "vite preview",
     "knip": "knip",
+    "lint": "eslint .",
+    "format": "prettier --write .",
+    "format:check": "prettier --check .",
     "test": "vitest run",
     "test:watch": "vitest",
     "e2e": "playwright test"
@@ -31,16 +34,24 @@
     "react-i18next": "^14.1.2"
   },
   "devDependencies": {
+    "@eslint/js": "9.39.1",
     "@playwright/test": "^1.61.1",
     "@types/node": "^26.1.1",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
     "autoprefixer": "^10.4.19",
+    "eslint": "9.39.1",
+    "eslint-config-prettier": "10.1.8",
+    "eslint-plugin-react-hooks": "5.2.0",
+    "eslint-plugin-react-refresh": "0.4.24",
+    "globals": "16.5.0",
     "knip": "6.27.0",
     "postcss": "^8.4.39",
+    "prettier": "3.6.2",
     "tailwindcss": "^3.4.6",
     "typescript": "^5.5.3",
+    "typescript-eslint": "8.46.0",
     "vite": "^5.3.4",
     "vitest": "^3.2.7"
   }