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/components/MessageList.css

@@ -0,0 +1,64 @@
+.message-list {
+  flex: 1;
+  overflow-y: auto;
+  padding: var(--sp-4);
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+}
+
+.message-row {
+  display: flex;
+  gap: var(--sp-2);
+  justify-content: flex-start;
+  align-items: flex-end;
+}
+
+.message-row-mine {
+  justify-content: flex-end;
+}
+
+.message-avatar-slot {
+  width: 28px;
+  flex: none;
+}
+
+.message-bubble-wrap {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  max-width: 65%;
+}
+
+.message-row-mine .message-bubble-wrap {
+  align-items: flex-end;
+}
+
+.message-author {
+  font-size: 0.76rem;
+  color: var(--ds-muted);
+  margin-bottom: 3px;
+  font-weight: 600;
+}
+
+.message-bubble {
+  background: var(--ds-surface-2);
+  color: var(--ds-text);
+  padding: 8px 12px;
+  border-radius: 10px;
+  font-size: 0.88rem;
+  line-height: 1.45;
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+
+.message-bubble-mine {
+  background: var(--ds-accent-2);
+}
+
+.message-time {
+  font-size: 0.68rem;
+  color: var(--ds-muted);
+  margin-top: 3px;
+  font-family: var(--mono);
+}