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

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>