Add GUI crate skeleton with libadwaita placeholder (build deferred until libgtk-4-dev/libadwaita-1-dev installed)

gui/Cargo.toml

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+[package]
+name = "ubuntu-fido-gui"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[[bin]]
+name = "ubuntu-fido"
+path = "src/main.rs"
+
+[dependencies]
+gtk = { package = "gtk4", version = "0.8" }
+adw = { package = "libadwaita", version = "0.6" }
+ubuntu-fido-common = { path = "../common" }
+anyhow = { workspace = true }

gui/data/io.dangerousthings.UbuntuFido.desktop

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+[Desktop Entry]
+Name=Authentication
+GenericName=Security Keys & MFA
+Comment=Manage U2F/FIDO2 keys and authentication policy
+Exec=ubuntu-fido
+Icon=io.dangerousthings.UbuntuFido
+Terminal=false
+Type=Application
+Categories=Settings;Security;
+Keywords=mfa;u2f;fido2;passkey;totp;security;
+StartupNotify=true

gui/data/io.dangerousthings.UbuntuFido.svg

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+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="64" height="64" viewBox="0 0 64 64">
+  <!-- Placeholder icon: a simple key glyph. Replace with branded asset before release. -->
+  <g fill="none" stroke="#3584e4" stroke-width="3" stroke-linecap="round" stroke-linejoin="round">
+    <circle cx="22" cy="32" r="9"/>
+    <path d="M31 32h22"/>
+    <path d="M44 32v8"/>
+    <path d="M52 32v6"/>
+  </g>
+</svg>

gui/src/main.rs

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+use adw::prelude::*;
+use gtk::glib;
+
+const APP_ID: &str = "io.dangerousthings.UbuntuFido";
+
+fn main() -> glib::ExitCode {
+    let app = adw::Application::builder().application_id(APP_ID).build();
+    app.connect_activate(|app| {
+        let win = adw::ApplicationWindow::builder()
+            .application(app)
+            .default_width(720)
+            .default_height(540)
+            .title("Authentication")
+            .build();
+        let header = adw::HeaderBar::new();
+        let toolbar = adw::ToolbarView::new();
+        toolbar.add_top_bar(&header);
+        let status = adw::StatusPage::builder()
+            .icon_name("dialog-password-symbolic")
+            .title("Authentication setup")
+            .description("Phase 0 placeholder. Real UI in Phase 8.")
+            .build();
+        toolbar.set_content(Some(&status));
+        win.set_content(Some(&toolbar));
+        win.present();
+    });
+    app.run()
+}