feat: serve the built Vue SPA from Quart with history-mode fallback

The frontend blueprint is registered after the api blueprint so /api/*
routes are matched first; everything else falls through to the SPA.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/__init__.py

@@ -6,6 +6,7 @@ from quart import Quart
 
 from .api import api_bp
 from .config import get_config
+from .frontend import frontend_bp
 
 
 def create_app() -> Quart:
@@ -15,5 +16,7 @@ def create_app() -> Quart:
     app = Quart(__name__)
     app.secret_key = cfg.secret_key
     app.register_blueprint(api_bp)
+    # Registered last so /api/* routes win over the SPA catch-all.
+    app.register_blueprint(frontend_bp)
 
     return app