feat(web): require Google login on every page, tag messages with user name

Auth is now mandatory for both the root dashboard and every workspace
page on agents.caremojo.app. The previous `?token=…` shortcut still
forwards the token to WorkspaceProvider once the user is signed in,
but it no longer bypasses authentication.

- Add `agents.caremojo.app` to `OPENAGENTS_HOSTNAMES` so Firebase
  Google OAuth activates on the production host.
- Root `/`: replace LandingPage with a centered LoginScreen card
  (logo + "Sign in to OpenAgents" + Google button) for unauthenticated
  visitors. Dashboard rendering for signed-in users is unchanged.
- `/[workspaceId]`: collapse the three-branch gate into a single
  "auth required" check. Token URL param is still threaded through to
  the WorkspaceProvider once authentication clears.
- Wire the signed-in user's display name into outgoing chat messages
  via the `source: human:<name>` field so other participants see the
  real sender instead of the generic "user" placeholder. Updates both
  ChatView and MonitorOverlay send paths.
- Render `message.senderName` for human messages in chat-message,
  falling back to "You" for legacy messages with no sender tag.