feat(ui): conversation-wide image gallery, layout fix, and session state restore

- Fix chat layout overflow when TaskPanel sidebar is open by adding min-w-0
  to flex containers in AppLayout and ChatPage
- Add conversation-wide image gallery with prev/next navigation (ArrowKeys)
  and image counter — collects images from both mediaItems and markdown
- Rename task panel header from "Tasks" to "Running Tasks"
- Add chat.session.status RPC to restore isRunning and activity state on
  session switch, preventing lost running indicators
- Add cancelled flag to session restore effect to prevent race conditions
  on rapid session switching

pkg/protocol/methods.go

@@ -11,10 +11,11 @@ const (
 	MethodAgentIdentityGet = "agent.identity.get"
 
 	// Chat
-	MethodChatSend    = "chat.send"
-	MethodChatHistory = "chat.history"
-	MethodChatAbort   = "chat.abort"
-	MethodChatInject  = "chat.inject"
+	MethodChatSend          = "chat.send"
+	MethodChatHistory       = "chat.history"
+	MethodChatAbort         = "chat.abort"
+	MethodChatInject        = "chat.inject"
+	MethodChatSessionStatus = "chat.session.status"
 
 	// Agents management
 	MethodAgentsList     = "agents.list"