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/services/invite_service.py

@@ -72,14 +72,15 @@ async def create_invite(
     db.add(invite)
     await db.commit()
     await db.refresh(invite)
+    invite.target_user = target
     return invite
 
 
 async def list_room_invites(db: AsyncSession, room_id: uuid.UUID) -> list[RoomInvite]:
     result = await db.execute(
-        select(RoomInvite).where(
-            RoomInvite.room_id == room_id, RoomInvite.status == InviteStatus.pending
-        )
+        select(RoomInvite)
+        .where(RoomInvite.room_id == room_id, RoomInvite.status == InviteStatus.pending)
+        .options(selectinload(RoomInvite.target_user))
     )
     return list(result.scalars().all())
 
@@ -92,7 +93,7 @@ async def list_my_invites(db: AsyncSession, user_id: uuid.UUID) -> list[RoomInvi
             RoomInvite.status == InviteStatus.pending,
             RoomInvite.expires_at > datetime.now(timezone.utc),
         )
-        .options(selectinload(RoomInvite.room))
+        .options(selectinload(RoomInvite.room), selectinload(RoomInvite.inviter))
     )
     return list(result.scalars().all())