feat(desktop): ask once what closing the window should do (cinny-desktop #5)

The first time the window is closed, a dialog asks "Keep Lotus Chat
running?" — Keep running in the tray (default, focused) / Quit when I
close the window — with an opt-in "Start Lotus Chat when I sign in"
checkbox in the same moment (per the approved design: one dialog, no
wizard). The choice is saved natively; Settings → General → "When I
close the window" changes it later (tray / quit / ask me).

The dialog is role="dialog" aria-modal, labelled and described, with
focus on the default button. Web-side flow verified with a stubbed
native side: event → dialog → checkbox + Quit sends autostart enable +
resolve_close_request("quit"); the Settings select saves "tray".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/settings/general/General.tsx

@@ -164,11 +164,18 @@ function AutostartSetting() {
   // [cinny-desktop #3] null until the native side answers (older builds don't
   // have the command, so the switch just stays hidden there).
   const [startMinimized, setStartMinimized] = useState<boolean | null>(null);
+  // [cinny-desktop #5] "ask" | "tray" | "quit"; null on builds without it.
+  const [closeBehavior, setCloseBehavior] = useState<string | null>(null);
 
   useEffect(() => {
     tauriInvoke()?.('plugin:autostart|is_enabled')
       .then((value) => setEnabled(value === true))
       .catch(() => undefined);
+    tauriInvoke()?.('get_close_behavior')
+      .then((value) => {
+        if (typeof value === 'string') setCloseBehavior(value);
+      })
+      .catch(() => undefined);
     tauriInvoke()?.('get_start_minimized')
       .then((value) => {
         if (typeof value === 'boolean') setStartMinimized(value);
@@ -181,6 +188,11 @@ function AutostartSetting() {
     setEnabled(value);
   };
 
+  const handleCloseBehavior = (value: string) => {
+    invokeTauri('set_close_behavior', { value });
+    setCloseBehavior(value);
+  };
+
   const handleStartMinimized = (value: boolean) => {
     invokeTauri('set_start_minimized', { value });
     setStartMinimized(value);
@@ -194,6 +206,24 @@ function AutostartSetting() {
         description="Start Lotus Chat automatically when you sign in to your computer."
         after={<Switch variant="Primary" value={enabled} onChange={handleChange} />}
       />
+      {closeBehavior !== null && (
+        <SettingTile
+          title="When I close the window"
+          description="Keep Lotus Chat running in the system tray (messages, notifications and calls keep arriving), or quit it. During a call, closing always keeps it running."
+          after={
+            <SettingsSelect
+              value={closeBehavior}
+              onChange={handleCloseBehavior}
+              aria-label="When I close the window"
+              options={[
+                { value: 'tray', label: 'Keep running in the tray' },
+                { value: 'quit', label: 'Quit Lotus Chat' },
+                { value: 'ask', label: 'Ask me' },
+              ]}
+            />
+          }
+        />
+      )}
       {enabled && startMinimized !== null && (
         <SettingTile
           title="Start minimized"