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/Composer.css

@@ -0,0 +1,44 @@
+.composer {
+  padding: var(--sp-3) var(--sp-4);
+  border-top: 1px solid var(--ds-border);
+  background: var(--ds-surface);
+  display: flex;
+  gap: var(--sp-2);
+  align-items: flex-end;
+}
+
+.composer textarea {
+  flex: 1;
+  resize: none;
+  background: var(--ds-surface-2);
+  border: 1px solid var(--ds-border);
+  border-radius: var(--radius);
+  padding: 9px 12px;
+  color: var(--ds-text);
+  font-size: 0.88rem;
+  outline: none;
+  max-height: 120px;
+}
+
+.composer textarea:focus {
+  border-color: var(--ds-accent);
+}
+
+.composer-send {
+  width: 36px;
+  height: 36px;
+  flex: none;
+  border-radius: var(--radius);
+  background: var(--ds-accent);
+  color: var(--ds-void);
+  border: none;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  cursor: pointer;
+}
+
+.composer-send:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}