feat(gui): Esc backs out one level

Pressing Esc on the Host/View screens stops the session and returns to the
menu (mirroring the '← Menu' button); on Settings it returns to the menu, or
closes the theme editor back to the picker if one is open. Suppressed while a
popup (colour picker, dropdown) is open so Esc just dismisses the popup there.

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

src/gui/mod.rs

@@ -942,6 +942,28 @@ impl PixelPassApp {
 
     /// Render the current screen. Called from inside the egui frame.
     fn draw(&mut self, ui: &mut egui::Ui) {
+        // Esc backs out one level, mirroring each screen's "← Menu" button: stop
+        // any session and return to the menu, or (in the theme editor) close the
+        // editor back to the picker first so a live preview isn't left applied.
+        // Suppressed while a popup (colour picker, dropdown) is open, so there
+        // Esc just dismisses the popup rather than also navigating.
+        let esc = ui.input(|i| i.key_pressed(egui::Key::Escape));
+        if esc && !ui.ctx().any_popup_open() {
+            match self.screen {
+                Screen::Host => {
+                    self.stop_host();
+                    self.screen = Screen::Menu;
+                }
+                Screen::Viewer => {
+                    self.stop_viewer();
+                    self.screen = Screen::Menu;
+                }
+                Screen::Settings if self.theme.editing => self.cancel_theme_edit(),
+                Screen::Settings => self.screen = Screen::Menu,
+                Screen::Menu => {}
+            }
+        }
+
         // Apply whichever theme should be visible this frame. While the editor
         // is open its draft is previewed live; otherwise the active theme is
         // applied once (when dirty) and then sticks — the egui context persists