feat(auth): redesign /register with OAuth buttons; hide invite-code under flag

Phase 2 Task 35. Adds OAuth Google/Microsoft sign-in to the register flow,
gated on the public SELF_SERVE_ENABLED flag, and hides the legacy invite-code
field when self-serve is on.

- New `useAppConfig` hook + `configApi`. One-shot module-cached fetch of
  `GET /api/v1/config/public`; falls back to `VITE_SELF_SERVE_ENABLED` env
  var (default false) if the endpoint is unreachable.
- New `OAuthCallbackPage` mounted at `/auth/google/callback` and
  `/auth/microsoft/callback` (public, NOT inside ProtectedRoute). Posts the
  authorization code to the backend, persists tokens, hydrates the auth
  store via fetchUser, and redirects to `/welcome` (new) or `/` (returning).
- `RegisterPage` now renders OAuth buttons + email/password divider when
  `self_serve_enabled` is true and only emits buttons for providers the
  backend reports as configured. Invite-code field hidden in that mode.
  Captures `?plan=pro` into `localStorage.rf-intended-plan` on mount.
- `authApi` gains `googleCallback(code)` / `microsoftCallback(code)`.
- `frontend/.env.example` + `frontend/Dockerfile` document and bake the
  three new VITE_* build-time variables (Lesson 60: Vite needs ARG+ENV).
- Vitest coverage for the three required cases plus the plan-param capture.

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

frontend/src/api/config.ts

@@ -0,0 +1,15 @@
+import apiClient from './client'
+
+export interface PublicConfig {
+  self_serve_enabled: boolean
+  oauth_providers: string[]
+}
+
+export const configApi = {
+  async getPublic(): Promise<PublicConfig> {
+    const response = await apiClient.get<PublicConfig>('/config/public')
+    return response.data
+  },
+}
+
+export default configApi