Rebuild frontend from the Claude Design handoff, DarkSingularity brand

Replaces the Phase 1 placeholder UI with a single persistent app shell (top
bar + sidebar + chat pane, 860px responsive breakpoint) matching the
"PWA chat system UI" design handoff: message bubbles with consecutive-run
avatar/name grouping, auto-growing composer, room search, and the real
DarkSingularity logo (also used to regenerate the PWA icons).

The handoff didn't cover Phase 2 (private rooms, roles, invites) or
browsing/joining open rooms, so those are added using the same visual
language: a room info panel with role badges, invite-by-username with a
pending-invites list, member management (remove/promote/demote/transfer
ownership), room settings (rename/describe/delete), and separate
browse-rooms/invites-inbox modals. Unread badges, last-message preview, and
the typing indicator are deliberately deferred -- both need new backend
features (read-tracking, a WS typing event) that weren't in scope this pass.

Two small backend additions round out data the new UI needs but the API
didn't expose: MessageRead.username (historic messages had no sender name)
and InviteRead.target_username / MyInviteRead.room_name+invited_by_username
(a recipient's invite list can't otherwise resolve a room they're not in).
Both are additive; 35 backend tests still pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas/invite.py

@@ -17,6 +17,15 @@ class InviteRead(BaseModel):
     room_id: uuid.UUID
     invited_by: uuid.UUID
     target_user_id: uuid.UUID | None
+    target_username: str | None = None
     status: InviteStatus
     expires_at: datetime
     created_at: datetime
+
+
+class MyInviteRead(InviteRead):
+    """InviteRead plus context the recipient can't otherwise resolve client-side --
+    GET /api/invites/mine is for rooms the user isn't a member of yet."""
+
+    room_name: str
+    invited_by_username: str

backend/app/schemas/message.py

@@ -10,5 +10,6 @@ class MessageRead(BaseModel):
     id: uuid.UUID
     room_id: uuid.UUID
     user_id: uuid.UUID
+    username: str
     content: str
     created_at: datetime