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/routes/dm.ts

@@ -695,6 +695,12 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       ))
       .run();
 
+    // Clean up read_states for the departing user
+    db.delete(schema.readStates).where(and(
+      eq(schema.readStates.userId, request.userId),
+      eq(schema.readStates.channelId, id),
+    )).run();
+
     // Check remaining members
     const remainingMembers = db.select()
       .from(schema.dmMembers)
@@ -743,6 +749,9 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
         });
       }
 
+      // Clean up all read_states for this DM channel (all members' rows)
+      db.delete(schema.readStates).where(eq(schema.readStates.channelId, id)).run();
+
       // Delete the DM channel (cascades to dm_messages)
       db.delete(schema.dmChannels).where(eq(schema.dmChannels.id, id)).run();