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/main.rs

@@ -1,5 +1,7 @@
 mod cli;
 mod common;
+#[cfg(feature = "gui")]
+mod gui;
 mod host;
 mod interactive;
 mod repair;
@@ -20,6 +22,20 @@ async fn main() -> Result<()> {
         common::output::set_json(true);
     }
 
+    if cli.gui {
+        #[cfg(feature = "gui")]
+        {
+            return gui::run();
+        }
+        #[cfg(not(feature = "gui"))]
+        {
+            anyhow::bail!(
+                "this binary was built without GUI support. Rebuild with \
+                 `cargo build --release --features gui` to use --gui."
+            );
+        }
+    }
+
     // libpipewire requires global init before any pw_* call. Idempotent;
     // safe to call even when the per-app audio thread never spawns.
     pipewire::init();