feat(web): scaffold React + Vite + TypeScript frontend

Phase 1 of frontend migration (epic #6, issue #7).

Project setup (web/):
- React 19, React Router 7, Vite 6, TypeScript 5.7
- Catppuccin Mocha theme CSS variables matching existing Go templates
- Vite dev proxy to Go backend at :8080 for /api/*, /login, /logout,
  /auth/*, /health, /ready

Shared infrastructure:
- api/client.ts: typed fetch wrapper (get/post/put/del) with 401
  redirect and credentials:include for session cookies
- api/types.ts: TypeScript interfaces for all API response types
  (User, Item, Project, Schema, Revision, BOMEntry, Audit, Error)
- context/AuthContext.tsx: AuthProvider calling GET /api/auth/me
- hooks/useAuth.ts: useAuth() hook exposing user/loading/logout

UI shell:
- AppShell.tsx: header nav matching current Go template navbar
  (Items, Projects, Schemas, Audit, Settings) with role badges
  (admin=mauve, editor=blue, viewer=teal) and active tab highlighting
- LoginPage: redirects to Go-served /login during transition
- Placeholder pages: Items, Projects, Schemas fetch from API and
  display data in tables; Audit shows summary stats; Settings shows
  current user profile

Go server changes:
- routes.go: serve web/dist/ at /app/* with SPA index.html fallback
  (only activates when web/dist/ directory exists)
- .gitignore: web/node_modules/, web/dist/
- Makefile: web-install, web-dev, web-build targets

web/vite.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    proxy: {
+      '/api': 'http://localhost:8080',
+      '/login': 'http://localhost:8080',
+      '/logout': 'http://localhost:8080',
+      '/auth': 'http://localhost:8080',
+      '/health': 'http://localhost:8080',
+      '/ready': 'http://localhost:8080',
+    },
+  },
+})