fix(ui): remember window size across launches

The window always opened at the hardcoded 900x760 because the size was never
persisted: run_gui hardcoded it, AppConfig had no size fields, and the Resized
handler only kept the size in memory (for divider clamping) while
exit_on_close_request:true quit before anything could save.

- AppConfig gains window_width/window_height (serde-default 900/760).
- run_gui restores them as the initial window size.
- The Resized handler mirrors the live size into config (guarded against bogus
  tiny sizes); divider positions on load now clamp against the restored size
  rather than a hardcoded default.
- exit_on_close_request:false + a CloseRequested handler writes the final size
  once, then iced::exit() — no per-resize disk thrash.

Verified empirically that KWin/Wayland honors a client-requested initial size
(requested 1150x680 -> window reported 1150x680). Window *position* is not
restored: xdg-shell gives Wayland clients no way to set their own position.

+1 config test (default + round-trip).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -92,6 +92,14 @@ fn default_chat_drawer_width() -> f32 {
     320.0
 }
 
+fn default_window_width() -> f32 {
+    900.0
+}
+
+fn default_window_height() -> f32 {
+    760.0
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct AppConfig {
     /// Last nickname used to join/create a room; pre-filled on the launch screen.
@@ -149,6 +157,12 @@ pub struct AppConfig {
     /// Empty / unset = look it up on `$PATH`. Hand-editable; no Settings UI yet.
     #[serde(default)]
     pub pixelpass_path: Option<String>,
+    /// Last window size (px), restored as the initial size on next launch.
+    /// Saved on close. (Window *position* can't be restored on Wayland.)
+    #[serde(default = "default_window_width")]
+    pub window_width: f32,
+    #[serde(default = "default_window_height")]
+    pub window_height: f32,
 }
 
 impl Default for AppConfig {
@@ -177,6 +191,8 @@ impl Default for AppConfig {
             custom_sound_mic_toggle: None,
             custom_sound_reconnect_failed: None,
             pixelpass_path: None,
+            window_width: default_window_width(),
+            window_height: default_window_height(),
         }
     }
 }
@@ -251,6 +267,27 @@ mod tests {
         assert!(deserialized.custom_sound_self_leave.is_none());
         assert!(deserialized.custom_sound_mic_toggle.is_none());
         assert!(deserialized.custom_sound_reconnect_failed.is_none());
+        // Configs predating the remembered window size load the default size.
+        assert_eq!(deserialized.window_width, 900.0);
+        assert_eq!(deserialized.window_height, 760.0);
+    }
+
+    #[test]
+    fn test_window_size_fields() {
+        // Default impl is the standard launch size.
+        let def = AppConfig::default();
+        assert_eq!(def.window_width, 900.0);
+        assert_eq!(def.window_height, 760.0);
+        // A saved size round-trips.
+        let cfg = AppConfig {
+            window_width: 1280.0,
+            window_height: 720.0,
+            ..AppConfig::default()
+        };
+        let json = serde_json::to_string(&cfg).unwrap();
+        let back: AppConfig = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.window_width, 1280.0);
+        assert_eq!(back.window_height, 720.0);
     }
 
     #[test]