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>

frontend/src/lib/messageGrouping.ts

@@ -0,0 +1,11 @@
+import type { RoomMember } from '../types'
+
+export function senderColorIndex(username: string, members: RoomMember[]): number {
+  const idx = members.findIndex((m) => m.username === username)
+  if (idx >= 0) return idx
+  // Fallback for a sender no longer in the room (e.g. they left): derive a
+  // stable index from the username instead of always colliding on 0.
+  let hash = 0
+  for (let i = 0; i < username.length; i++) hash = (hash * 31 + username.charCodeAt(i)) | 0
+  return Math.abs(hash)
+}