perf(P6C.T12): optimistic UI for mute / mentions-only / archive / pin / device-revoke

Audit of write-actions revealed that send (and edit via realtime UPDATE) are
already optimistic via local state insertion in `useConversationMessages`,
and friend nicknames are pure-local localStorage. Five user-write actions
were waiting on the ~100-200 ms server roundtrip + realtime echo:

* Toggle mute (`setConversationMutedUntil`)
* Toggle mentions-only (`setConversationMentionsOnly`)
* Toggle archive (`setConversationArchived`)
* Pin / unpin message (`pinMessage` / `unpinMessage`)
* Revoke device (`revokeDevice` RPC)

All five now flip local state synchronously and roll back on failure. The
existing realtime subscriptions reconcile canonically (no-op when the
optimistic patch already matches the server row), so this is purely a UX
latency improvement — no protocol or persistence changes.

Reactions (`toggleReaction` / `voteExclusive`) were intentionally skipped
this round: rollback semantics for the exclusive-vote path with multiple
sequential awaits are messy enough to warrant a dedicated pass.

apps/desktop/src/context/ConversationsContext.tsx

@@ -54,6 +54,15 @@ interface ConversationsContextValue {
   refresh: () => Promise<void>;
   markRead: (conversationId: string) => void;
   setActiveConversation: (conversationId: string | null) => void;
+  // Optimistic patch for the caller's per-membership preferences (mute /
+  // mentions-only / archive). Mutations to `conversation_members` echo back
+  // via the realtime channel and `refresh()` reconciles canonically, but the
+  // ~100-200ms roundtrip leaves the UI looking unresponsive. Callers patch
+  // immediately, snapshot the previous state, and roll back on failure.
+  patchConversation: (
+    conversationId: string,
+    patch: Partial<Pick<ConversationSummary, 'archived' | 'mutedUntil' | 'mentionsOnly'>>,
+  ) => void;
 }
 
 const ConversationsContext = createContext<ConversationsContextValue | null>(null);
@@ -164,6 +173,19 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
     [markRead],
   );
 
+  const patchConversation = useCallback<ConversationsContextValue['patchConversation']>(
+    (convId, patch) => {
+      setConversations((prev) => {
+        const idx = prev.findIndex((c) => c.id === convId);
+        if (idx === -1) return prev;
+        const next = [...prev];
+        next[idx] = { ...next[idx]!, ...patch };
+        return next;
+      });
+    },
+    [],
+  );
+
   useEffect(() => {
     if (!myId) {
       setConversations([]);
@@ -308,6 +330,7 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
       refresh,
       markRead,
       setActiveConversation,
+      patchConversation,
     }),
     [
       conversations,
@@ -318,6 +341,7 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
       refresh,
       markRead,
       setActiveConversation,
+      patchConversation,
     ],
   );