chore: add Biome for lint and fix all lint errors

- Install @biomejs/biome as devDependency at root
- Configure biome.json with 2-space indent, double quotes, Tailwind CSS support
- Add lint/lint:fix/format/format:fix scripts to root and app package.json
- Fix noNonNullAssertion: env vars extracted to variables with suppression, <div role=button> replaced with <button>
- Fix noUnusedVariables: remove unused destructured vars
- Fix useIterableCallbackReturn: arrow functions with block body
- Fix noExplicitAny: recharts Tooltip formatters
- Fix noLabelWithoutControl: add htmlFor+id or use <span> for non-input labels
- Fix noStaticElementInteractions/useKeyWithClickEvents: role+keyboard events for overlays
- Fix noArrayIndexKey: use error string as key
- Fix CSS parse: enable tailwindDirectives parser
- Normalize formatting across 80 files with biome check --write

package.json

@@ -3,13 +3,18 @@
   "private": true,
   "scripts": {
     "dev": "conc -n backend,frontend -c blue,green \"bun run --cwd apps/backend dev\" \"bun run --cwd apps/frontend dev\"",
-    "build": "bun run --cwd apps/frontend build"
+    "build": "bun run --cwd apps/frontend build",
+    "lint": "biome check .",
+    "lint:fix": "biome check --write .",
+    "format": "biome format .",
+    "format:fix": "biome format --write ."
   },
   "workspaces": [
     "packages/*",
     "apps/*"
   ],
   "devDependencies": {
+    "@biomejs/biome": "^2.4.16",
     "concurrently": "^9.1.0"
   },
   "dependencies": {