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.

backend/.env.example

@@ -1,12 +1,13 @@
 # Copy to .env and fill in. Never commit .env.
 
 # --- Nextcloud ---
+# Server address only - each user logs in themselves via Nextcloud Login
+# Flow v2 (see /auth/login/start); there's no shared NC_USERNAME/APP_PASSWORD
+# any more.
 NC_BASE_URL=https://cloud.dominik-roth.eu
-NC_USERNAME=dodox
-# Settings -> Security -> "Devices & Sessions" -> create app password
-NC_APP_PASSWORD=
-# Folder (relative to the user's files root) receipt images get uploaded to.
-# Created automatically on first upload if missing (each path segment).
+# Folder (relative to each user's own files root) receipt images get
+# uploaded to. Created automatically on first upload if missing (each path
+# segment).
 NC_UPLOAD_FOLDER=Documents/Cospend/Assets
 
 # --- Cospend ---
@@ -29,3 +30,15 @@ LLM_MODEL=
 # --- Flask ---
 FLASK_SECRET_KEY=dev-change-me
 DATABASE_PATH=instance/wgbill.sqlite3
+
+# --- Auth ---
+# Generate with: python3 -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+# Encrypts stored Nextcloud app passwords at rest. Required - the app
+# refuses to store/read a token without it. Keep this secret; losing it
+# means every logged-in user has to log in again.
+TOKEN_ENCRYPTION_KEY=
+# Idle session lifetime in days (sliding - refreshed on each use).
+SESSION_TTL_DAYS=30
+# Cookies need Secure (HTTPS-only) for any real deployment. Only set to
+# false for plain-http localhost dev.
+SESSION_COOKIE_SECURE=true