Per-user auth via Nextcloud Login Flow v2, Tailwind UI rewrite

- auth.py/auth_routes.py: Login Flow v2 (verified against real NC
  source/docs) - no OAuth2 client registration needed, backend polls
  server-side so no CORS issues. Sessions are hashed-token cookies;
  NC app passwords encrypted at rest (Fernet). Every /api/* route
  guarded by a blueprint-wide before_request, not per-route decorators,
  so future routes are protected by default.
- receipts/groups scoped per owner_nc_user_id; cross-user access 404s.
- nc_client/cospend_client take (username, app_password) per call
  instead of one shared global credential - each user's uploads/shares/
  bills now happen as themselves.
- Frontend: LoginGate component drives the login flow (open NC login
  in a new tab, poll our backend, done).
- Merged the old separate review step into the split screen, redesigned
  with Tailwind (was unstyled/broken), default-excluded-per-item
  splitting with one-click "include everyone" fixed, DD.MM.YYYY date
  field, receipt-icon branding.
- Dropped LLM bounding-box highlighting - unreliable on real receipts,
  plain photo upload instead.
- Only mention who a bill is split with in its title when there's more
  than one bill off the same receipt to disambiguate.

frontend/vite.config.ts

@@ -1,3 +1,4 @@
+import tailwindcss from '@tailwindcss/vite'
 import react from '@vitejs/plugin-react'
 import { defineConfig } from 'vite'
 import { VitePWA } from 'vite-plugin-pwa'
@@ -6,6 +7,7 @@ import { VitePWA } from 'vite-plugin-pwa'
 export default defineConfig({
   plugins: [
     react(),
+    tailwindcss(),
     VitePWA({
       registerType: 'autoUpdate',
       manifest: {
@@ -25,6 +27,7 @@ export default defineConfig({
   server: {
     proxy: {
       '/api': 'http://localhost:5000',
+      '/auth': 'http://localhost:5000',
     },
   },
 })