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/tests/test_invites.py

@@ -49,6 +49,11 @@ async def test_invite_accept_flow(client, db_session):
     assert resp.status_code == 201
     invite = resp.json()
     assert invite["status"] == "pending"
+    assert invite["target_username"] == "bob"
+
+    resp = await client.get(f"/api/rooms/{room['id']}/invites")
+    assert resp.status_code == 200
+    assert resp.json()[0]["target_username"] == "bob"
 
     await client.post("/api/auth/logout")
     await login_as(client, "bob")
@@ -58,6 +63,8 @@ async def test_invite_accept_flow(client, db_session):
     mine = resp.json()
     assert len(mine) == 1
     assert mine[0]["id"] == invite["id"]
+    assert mine[0]["room_name"] == room["name"]
+    assert mine[0]["invited_by_username"] == "alice"
 
     resp = await client.post(f"/api/invites/{invite['id']}/accept")
     assert resp.status_code == 200