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>

Cargo.toml

@@ -33,8 +33,14 @@ arboard = { version = "3", default-features = false, features = ["wayland-data-c
 ureq = { version = "3", default-features = false, features = ["rustls"] }
 toml = "1"
 chrono = { version = "0.4", default-features = false, features = ["clock", "serde"] }
+eframe = { version = "0.34.2", default-features = false, features = ["glow", "default_fonts", "wayland", "x11"], optional = true }
 
 [profile.release]
 lto = "thin"
 codegen-units = 1
 strip = "symbols"
+
+[features]
+# Opt-in graphical front-end (pixelpass --gui). Default-off so the headless
+# build never pulls the GUI toolkit tree.
+gui = ["dep:eframe"]