feat(frontend): closed-channel read-only UI

Closed channels: composer replaced by a read-only banner; history still
viewable; GuildChannel carries closed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.css

@@ -683,6 +683,17 @@ button {
 .dc-composer .btn {
   flex: none;
 }
+.dc-closed-banner {
+  flex: none;
+  margin: 0 16px 20px;
+  padding: 12px 14px;
+  text-align: center;
+  color: var(--text-muted);
+  background: var(--elevated);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  font-size: 13px;
+}
 
 /* members */
 .dc-members {

src/pages/ChatPage.tsx

@@ -34,7 +34,7 @@ function timeOf(iso?: string): string {
 const X_TYPES = ['general', 'work', 'report', 'discuss', 'triage', 'custom'] as const
 type XType = (typeof X_TYPES)[number]
 
-type GuildChannel = { id: string; name: string; guildId?: string; xType?: XType; isMember?: boolean; isPublic?: boolean }
+type GuildChannel = { id: string; name: string; guildId?: string; xType?: XType; isMember?: boolean; isPublic?: boolean; closed?: boolean }
 type MemberItem = { userId: string; email: string; name: string; status: string }
 
 export default function ChatPage() {
@@ -500,6 +500,9 @@ export default function ChatPage() {
           ))}
         </div>
 
+        {currentChannel?.closed ? (
+          <div className="dc-closed-banner">This channel is closed — history is read-only.</div>
+        ) : (
           <form
             className="dc-composer"
             onSubmit={(e) => {
@@ -518,6 +521,7 @@ export default function ChatPage() {
               Send
             </button>
           </form>
+        )}
       </main>
 
       {showMembers ? (