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/routers/rooms.py

@@ -257,7 +257,31 @@ async def get_room_messages_endpoint(
     db: AsyncSession = Depends(get_db),
 ):
     await require_room_member(room_id, current_user, db)
-    return await list_recent_messages(db, room_id, limit)
+    messages = await list_recent_messages(db, room_id, limit)
+    return [
+        MessageRead(
+            id=m.id,
+            room_id=m.room_id,
+            user_id=m.user_id,
+            username=m.user.username,
+            content=m.content,
+            created_at=m.created_at,
+        )
+        for m in messages
+    ]
+
+
+def _to_invite_read(invite) -> InviteRead:
+    return InviteRead(
+        id=invite.id,
+        room_id=invite.room_id,
+        invited_by=invite.invited_by,
+        target_user_id=invite.target_user_id,
+        target_username=invite.target_user.username if invite.target_user else None,
+        status=invite.status,
+        expires_at=invite.expires_at,
+        created_at=invite.created_at,
+    )
 
 
 @router.post("/{room_id}/invites", response_model=InviteRead, status_code=201)
@@ -269,13 +293,14 @@ async def create_invite_endpoint(
 ):
     await require_room_role(room_id, current_user, db, RoomRole.admin)
     try:
-        return await create_invite(db, room_id, current_user.id, data.target_username)
+        invite = await create_invite(db, room_id, current_user.id, data.target_username)
     except TargetUserNotFoundError:
         raise HTTPException(status_code=404, detail="No user with that username")
     except AlreadyMemberError:
         raise HTTPException(status_code=409, detail="That user is already a member")
     except DuplicateInviteError:
         raise HTTPException(status_code=409, detail="That user already has a pending invite")
+    return _to_invite_read(invite)
 
 
 @router.get("/{room_id}/invites", response_model=list[InviteRead])
@@ -285,7 +310,8 @@ async def list_room_invites_endpoint(
     db: AsyncSession = Depends(get_db),
 ):
     await require_room_role(room_id, current_user, db, RoomRole.admin)
-    return await list_room_invites(db, room_id)
+    invites = await list_room_invites(db, room_id)
+    return [_to_invite_read(i) for i in invites]
 
 
 @router.delete("/{room_id}/invites/{invite_id}", status_code=204)