feat(gui): scaffold egui window behind the gui feature

Adds the opt-in graphical front-end (pixelpass --gui), default-off via the
`gui` cargo feature so the headless build never pulls the toolkit tree.
eframe 0.34 on the glow/OpenGL backend (no wgpu); 69 feature-gated crates,
vetted. --gui on a headless build errors with a rebuild hint.

This commit is just the shell: a window with a Host/View menu and back
navigation. The shell-out child-spawning + JSON event parsing that drives
real host/viewer controls come next. Window verified to open and render
cleanly on Wayland (glow).

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

src/gui/mod.rs

@@ -0,0 +1,106 @@
+//! Graphical front-end (`pixelpass --gui`), compiled only with the `gui`
+//! feature.
+//!
+//! Architecture: this window is a thin **shell-out** driver. It never touches
+//! the capture / portal / gst / iroh machinery directly — instead it re-execs
+//! this same binary in headless mode (`pixelpass --host --output json …` or
+//! `pixelpass <ticket> --output json`) as a child process and parses the
+//! child's JSON event stream (see [`crate::common::output`]) to drive what it
+//! shows. That keeps the fragile capture stack sealed in a separate process:
+//! the GUI can be closed or crash without taking a live stream down.
+
+use eframe::egui;
+
+/// Launch the GUI event loop. Blocks until the window is closed. Runs on the
+/// main thread (a winit requirement), which is where `main` calls it from.
+pub fn run() -> anyhow::Result<()> {
+    let options = eframe::NativeOptions {
+        viewport: egui::ViewportBuilder::default()
+            .with_inner_size([520.0, 420.0])
+            .with_min_inner_size([420.0, 320.0])
+            .with_title("PixelPass"),
+        ..Default::default()
+    };
+
+    eframe::run_native(
+        "PixelPass",
+        options,
+        Box::new(|_cc| Ok(Box::new(PixelPassApp::default()))),
+    )
+    .map_err(|e| anyhow::anyhow!("GUI failed to start: {e}"))
+}
+
+/// Which screen the single window is currently showing.
+#[derive(Default, PartialEq)]
+enum Screen {
+    #[default]
+    Menu,
+    Host,
+    Viewer,
+}
+
+#[derive(Default)]
+struct PixelPassApp {
+    screen: Screen,
+}
+
+impl eframe::App for PixelPassApp {
+    // eframe 0.34 hands us the central-panel `ui` directly (it wraps this in a
+    // CentralPanel for us).
+    fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
+        match self.screen {
+            Screen::Menu => self.menu(ui),
+            Screen::Host => self.host(ui),
+            Screen::Viewer => self.viewer(ui),
+        }
+    }
+}
+
+impl PixelPassApp {
+    fn menu(&mut self, ui: &mut egui::Ui) {
+        ui.vertical_centered(|ui| {
+            ui.add_space(24.0);
+            ui.heading("PixelPass");
+            ui.label("P2P screen sharing");
+            ui.add_space(32.0);
+            if ui
+                .add_sized([240.0, 40.0], egui::Button::new("Host — share my screen"))
+                .clicked()
+            {
+                self.screen = Screen::Host;
+            }
+            ui.add_space(8.0);
+            if ui
+                .add_sized(
+                    [240.0, 40.0],
+                    egui::Button::new("View — watch someone's screen"),
+                )
+                .clicked()
+            {
+                self.screen = Screen::Viewer;
+            }
+        });
+    }
+
+    fn host(&mut self, ui: &mut egui::Ui) {
+        self.back_bar(ui, "Host");
+        ui.separator();
+        ui.label("Host controls land in the next step.");
+    }
+
+    fn viewer(&mut self, ui: &mut egui::Ui) {
+        self.back_bar(ui, "View");
+        ui.separator();
+        ui.label("Viewer controls land in a later step.");
+    }
+
+    /// Title row with a back-to-menu button, shared by the host/viewer screens.
+    fn back_bar(&mut self, ui: &mut egui::Ui, title: &str) {
+        ui.horizontal(|ui| {
+            if ui.button("← Menu").clicked() {
+                self.screen = Screen::Menu;
+            }
+            ui.heading(title);
+        });
+    }
+}