feat(frontend): scaffold — Tailwind v4, routing, API layer, UserContext

- tailwindcss + @tailwindcss/vite, react-router-dom, date-fns, zod installed
- vite.config.ts: Tailwind v4 Vite plugin wired in
- index.css: single @import "tailwindcss" (zero-config v4)
- main.tsx: wrapped in <BrowserRouter>; imports index.css
- App.tsx: layout shell with <TopBar/> + <Routes> (/, /weekly, /monthly, /projects)
- lib/api.ts: Zod schemas for all API types + typed fetch wrapper; all
  responses validated before field access (per CLAUDE.md rule)
- lib/format.ts: minutesToHmm, hmmToMinutes, centsToEur, ISO week helpers,
  year-month helpers, formatDate
- lib/types.ts: z.infer<> re-exports from api.ts
- state/UserContext.tsx: selected user persisted in localStorage; auto-
  selects first user on load

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/package.json

@@ -13,12 +13,15 @@
     "format": "biome format --write src"
   },
   "dependencies": {
+    "date-fns": "^4.1.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
+    "react-router-dom": "^7.15.1",
     "zod": "^3.23.8"
   },
   "devDependencies": {
     "@biomejs/biome": "^1.9.4",
+    "@tailwindcss/vite": "^4.3.0",
     "@testing-library/jest-dom": "^6.6.3",
     "@testing-library/react": "^16.1.0",
     "@testing-library/user-event": "^14.5.2",
@@ -26,6 +29,7 @@
     "@types/react-dom": "^19.0.2",
     "@vitejs/plugin-react": "^4.3.4",
     "jsdom": "^25.0.1",
+    "tailwindcss": "^4.3.0",
     "typescript": "~5.7.2",
     "vite": "^6.0.5",
     "vitest": "^2.1.8"