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

apps/backend/package.json

@@ -9,7 +9,9 @@
     "prisma:migrate": "prisma migrate dev",
     "prisma:studio": "prisma studio",
     "prisma:migrate:deploy": "prisma migrate deploy",
-    "prisma:seed": "bun run prisma/seed.ts"
+    "prisma:seed": "bun run prisma/seed.ts",
+    "lint": "biome check src/",
+    "lint:fix": "biome check --write src/"
   },
   "dependencies": {
     "better-auth": "^1.6.11",