gui: persist prompt draft text per session

When switching between sessions, the draft text in the prompt
is saved and restored. Each pane maintains a _drafts map keyed
by session ID, so switching away and back preserves whatever
was typed.

gui/ChatPane.qml

@@ -23,13 +23,25 @@ Item {
     property string paneCwd: ""
     property int chatOffset: 0
 
+    // Per-session draft text storage
+    property var _drafts: ({})
+    property string _prevSessionId: ""
+
     // Fetch chat when session changes
     onSessionIdChanged: {
+        // Save draft from previous session
+        if (_prevSessionId !== "") {
+            _drafts[_prevSessionId] = panePrompt.text
+        }
+
         paneModel.rawText = ""
         chatOffset = 0
         paneState = "idle"
         paneCwd = ""
         if (sessionId !== "") {
+            // Restore draft for new session (or empty)
+            panePrompt.text = _drafts[sessionId] || ""
+
             // Fetch initial chat and config
             let chat = dbus.getChat(sessionId, 0)
             if (chat) {
@@ -40,7 +52,10 @@ Item {
             if (cfg && cfg.cwd) paneCwd = cfg.cwd
             let st = dbus.getState(sessionId)
             if (st) paneState = st
+        } else {
+            panePrompt.text = ""
         }
+        _prevSessionId = sessionId
     }
 
     // Listen for D-Bus signals filtered to our session