gui: Settings toggle to hide the host QR-code panel

Adds a `show_qr` preference (default on) to GuiSettings, with a
checkbox in Settings and a corresponding gate on the host-screen render.
Persists to config.toml alongside the existing close-to-tray setting.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/common/config.rs

@@ -20,13 +20,31 @@ pub struct Config {
 }
 
 /// Preferences for the `pixelpass --gui` front-end.
-#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct GuiSettings {
     /// When true, the window's close button hides the app to the system tray
     /// (keeping any live stream running) instead of quitting. Defaults to
     /// false — closing quits, which is what people expect.
     #[serde(default)]
     pub close_to_tray: bool,
+    /// When true, the host screen renders a QR-code panel for the ticket.
+    /// Defaults to true; the toggle exists for users who prefer the plain
+    /// text-only host screen.
+    #[serde(default = "default_true")]
+    pub show_qr: bool,
+}
+
+impl Default for GuiSettings {
+    fn default() -> Self {
+        Self {
+            close_to_tray: false,
+            show_qr: true,
+        }
+    }
+}
+
+fn default_true() -> bool {
+    true
 }
 
 /// Result of the first-run upstream measurement.