Add composer autocomplete for #roomname (#47 follow-up)

Mirrors the @mention autocomplete exactly -- same trigger-detection
logic (factored into a shared detectTriggerQuery helper, parameterized
on '@' vs '#'), same arrow-key/Enter/Tab/Escape keyboard handling, same
dropdown. Suggests rooms the user belongs to, filtered by name prefix,
showing the room's description as a subtitle when it has one.

MentionAutocomplete.css is renamed to ComposerAutocomplete.css with
generic class names (composer-autocomplete-primary/-secondary instead of
-username/-display-name), now shared by both the mention and room-
reference dropdowns instead of being mention-specific.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/ComposerAutocomplete.css

@@ -0,0 +1,49 @@
+.composer-autocomplete {
+  position: absolute;
+  bottom: calc(100% + 8px);
+  left: 0;
+  z-index: 31;
+  width: 240px;
+  max-height: 220px;
+  overflow-y: auto;
+  background: var(--ds-surface);
+  border: 1px solid var(--ds-border);
+  border-radius: var(--radius);
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.35);
+  padding: 4px;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+
+.composer-autocomplete-item {
+  display: flex;
+  align-items: baseline;
+  gap: 6px;
+  background: transparent;
+  border: none;
+  border-radius: 6px;
+  padding: 6px 8px;
+  text-align: left;
+  cursor: pointer;
+  color: var(--ds-text);
+}
+
+.composer-autocomplete-item-active,
+.composer-autocomplete-item:hover {
+  background: var(--ds-surface-2);
+}
+
+.composer-autocomplete-primary {
+  font-size: 0.84rem;
+  font-weight: 700;
+  color: var(--ds-accent);
+}
+
+.composer-autocomplete-secondary {
+  font-size: 0.76rem;
+  color: var(--ds-muted);
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}