Finish Phase 6: Push frontend, WebAuthn frontend, OpenAPI/Swagger

Frontend:
- stores/push.ts: fixed VAPID key endpoint, added checkSubscription, proper subscription format
- UserSettings.tsx: added [ENABLE PUSH] toggle and [REGISTER PASSKEY] button
- LoginForm.tsx: added [SIGN IN WITH PASSKEY] button

Backend:
- cmd/server/main.go: added Swagger annotations, /docs/* route
- docs/: generated swagger.json, swagger.yaml, docs.go
- Makefile: added docs target for swag init

Makefile

@@ -11,6 +11,10 @@ build-server:
 build-tui:
 	CGO_ENABLED=0 go build -o dumpster-tui ./cmd/tui
 
+# Generate Swagger docs
+docs:
+	~/go/bin/swag init -g cmd/server/main.go -o docs
+
 # Build the web frontend
 build-web:
 	cd web && npm run build