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.lock

@@ -4176,6 +4176,7 @@ dependencies = [
  "nix 0.30.1",
  "notify-rust",
  "pipewire",
+ "qrcode",
  "serde",
  "serde_json",
  "thiserror 2.0.18",
@@ -4420,6 +4421,12 @@ version = "0.1.29"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e0c5ccf5294c6ccd63a74f1565028353830a9c2f5eb0c682c355c471726a6e3f"
 
+[[package]]
+name = "qrcode"
+version = "0.14.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d68782463e408eb1e668cf6152704bd856c78c5b6417adaee3203d8f4c1fc9ec"
+
 [[package]]
 name = "quick-error"
 version = "2.0.1"