feat: standalone mode — FlowDeck works without any Git forge

- config.py: FLOWDECK_STANDALONE flag (default false)
- dashboard: graceful fallback when Gitea API unavailable
  Works without GITEA_TOKEN or with FLOWDECK_STANDALONE=true
- Login page already supports local-only mode
- Workspace page: forge sections hidden when no projects
- Application fully functional with zero external dependencies

app/config.py

@@ -21,6 +21,9 @@ class Settings(BaseSettings):
     github_client_id: str = ""
     github_client_secret: str = ""
 
+    # Standalone mode
+    standalone: bool = False  # FLOWDECK_STANDALONE=true in .env
+
     # OAuth2
     oauth_redirect_uri: str = "http://localhost:8080/auth/callback"