fix: eliminate phantom notifications across the entire read-state pipeline

Root cause: own messages echoed by the server marked channels unread when
the user had already navigated away. Seven related bugs compounded the
problem — stale read states, missing cleanup on space/DM removal, REST
broadcast ignoring VIEW_CHANNEL, and no validation on channel_ack writes.

Frontend:
- Skip markChannelUnread for the user's own messages (federation-aware)
- Walk backward past temp_ IDs in ackChannel instead of bailing
- Re-fire ack timer when temp message is replaced by server-confirmed ID
- Add removeChannelStates to clean up unread/read/message caches
- Clean up chatStore on removeSpace, removeDmChannel, removeInstanceSpaces

Server:
- Use sendToChannel instead of sendToSpace for REST message creation
- Clean up read_states on space deletion, member kick/leave, and ban
- Validate channel membership before accepting channel_ack writes
- Clean up read_states on DM leave and DM channel deletion

packages/server/src/ws/events.ts

@@ -1040,6 +1040,14 @@ function handleChannelAck(event: Record<string, unknown>, userId: string): void
   // Validate messageId is a valid snowflake (numeric string) — reject temp/garbage IDs
   if (!/^\d+$/.test(messageId)) return;
 
+  // Validate channel membership — reject acks for channels the user doesn't belong to
+  const spaceId = getChannelSpaceId(channelId);
+  if (spaceId) {
+    if (!isMember(spaceId, userId)) return;
+  } else {
+    if (!isDmMember(channelId, userId)) return;
+  }
+
   const db = getDb();
 
   const existing = db.select()