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

docs/swagger.yaml

@@ -0,0 +1,17 @@
+basePath: /api/v1
+host: localhost:8080
+info:
+  contact: {}
+  description: A chaotic, self-hosted Discord-like platform API
+  title: Dumpster API
+  version: "1.0"
+paths: {}
+schemes:
+- http
+- https
+securityDefinitions:
+  SessionAuth:
+    in: cookie
+    name: dumpster_session
+    type: apiKey
+swagger: "2.0"