gui: QR-code panel for the host ticket

Encodes the relay-only ticket as a QR with a 4-module quiet zone so a
phone (or a second laptop with a webcam) can pick the room up without
typing 140+ characters. Built lazily on the first draw after a Ticket
event, NEAREST-filtered, 200x200 logical; cleared on session start and
stop.

Pulls `qrcode` 0.14 with `default-features = false` so the heavy `image`
crate tree is skipped — we render modules straight to an
`egui::ColorImage` ourselves.

Reapplies the idea from Gemini's stale `feat/gemini-branch-qrcode`
(`7f07583`) against the post-hand-rolled-loop GUI; the original commit
no longer cherry-picks because gui/mod.rs was rewritten for the
true-Wayland-window-hide work.

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

Cargo.toml

@@ -55,6 +55,11 @@ egui_glow = { version = "0.34.2", default-features = false, features = ["winit",
 winit = { version = "0.30", default-features = false, features = ["rwh_06", "x11", "wayland", "wayland-dlopen"], optional = true }
 glutin = { version = "0.32", optional = true }
 glutin-winit = { version = "0.5", optional = true }
+# QR-encode the host ticket so a phone (or a second laptop with a webcam) can
+# pick it up without typing 140 chars. default-features = false to skip the
+# `image` crate dep tree — we render the modules to an `egui::ColorImage`
+# directly.
+qrcode = { version = "0.14", default-features = false, optional = true }
 
 [profile.release]
 lto = "thin"
@@ -64,4 +69,4 @@ 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", "dep:notify-rust", "dep:ksni", "dep:egui_glow", "dep:winit", "dep:glutin", "dep:glutin-winit"]
+gui = ["dep:eframe", "dep:notify-rust", "dep:ksni", "dep:egui_glow", "dep:winit", "dep:glutin", "dep:glutin-winit", "dep:qrcode"]