feat(gui): TouchDialog modal racing EnrollOwn against EnrollmentFailed signal

Adds enroll_dialog::present which opens an adw::AlertDialog with a spinner
and races the EnrollOwn call against the EnrollmentFailed signal stream:
- Call returns Ok(_cred) -> close dialog, fire on_success refresh callback.
- Call returns Err -> swap dialog body to the user-message form.
- Signal arrives first -> swap to the signal payload (faster than waiting
  for the call's typed Err to traverse the bus).

KeysPage::start_enroll wires the activated row to present(), passing the
parent window for modal anchoring and a refresh closure as on_success.

Plan deviation: AlertDialog is libadwaita v1_5-gated, so gui/Cargo.toml
enables that feature. Targets Ubuntu 24.04+ (libadwaita 1.5).

gui/Cargo.toml

@@ -10,7 +10,7 @@ path = "src/main.rs"
 
 [dependencies]
 gtk = { package = "gtk4", version = "0.8" }
-adw = { package = "libadwaita", version = "0.6" }
+adw = { package = "libadwaita", version = "0.6", features = ["v1_5"] }
 authforge-common = { path = "../common" }
 anyhow = { workspace = true }
 zbus = { workspace = true }