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/frontend/src/components/layout/Header.tsx

@@ -1,8 +1,8 @@
-import { Menu, Sun, Moon, Monitor, LogOut } from "lucide-react";
+import { LogOut, Menu, Monitor, Moon, Sun } from "lucide-react";
 import { Button } from "@/components/ui/button";
+import { useAuth } from "@/lib/auth-context";
 import { useSseStatus } from "@/lib/sse-context";
 import { useTheme } from "@/lib/theme";
-import { useAuth } from "@/lib/auth-context";
 import { cn } from "@/lib/utils";
 
 interface HeaderProps {