Add app icons, screenshots, and complete AppStream metainfo

- Add GNOME HIG-compliant app icon (scalable SVG) and symbolic variant
- Add 12 screenshots covering all major views and features
- Flesh out metainfo with screenshots, categories, URLs, content
  rating, system requirements, provides, translation, donation
  and contact links
- Update AppImage build script to bundle GTK plugin, symbolic
  icon, and metainfo
- Update meson.build with icon installation rules
- Remove About dialog from application menu
- Remove unused user guide and audit tool

data/icons/hicolor/scalable/apps/app.driftwood.Driftwood.svg

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="128" height="128" viewBox="0 0 128 128">
+  <!-- Chin/depth layer (GNOME Green 5) -->
+  <rect x="10" y="14" width="108" height="104" rx="24" fill="#26a269" />
+  <!-- Main face (GNOME Green 4) -->
+  <rect x="10" y="10" width="108" height="104" rx="24" fill="#2ec27e" />
+  <!-- Driftwood symbol -->
+  <g transform="translate(-8, -10) scale(1.44)">
+    <path fill="none" stroke="white" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" stroke-miterlimit="10" d="M52.104,56.311c0-3.775,3.061-6.836,6.836-6.836c3.777,0,6.836,3.061,6.836,6.836 M62.293,55.785c0-1.652-1.34-2.99-2.994-2.99c-1.65,0-2.988,1.338-2.988,2.99 M48.948,56.311h19.983 M59.789,46.319l-13.996-1.577c-3.155-1.053-4.207-7.363-4.207-7.363l-3.154,2.104c0,0,2.103,4.207-1.053,4.207l-5.784-0.525c-3.194,0-5.784,2.59-5.784,5.785l23.137,7.361c0-5.519,4.474-9.991,9.991-9.991c5.52,0,9.992,4.473,9.992,9.991 M45.793,50l-6.311-3.154l-9.465-1.053 M35.276,48.949L39.482,50 M37.379,59.466l10.518,3.155h10.517 M74.189,56.311H80.5l-8.414-3.155 M64.725,59.466H53.156 M31.068,57.362l-6.31-2.104 M19.5,50l18.932,6.311" />
+  </g>
+</svg>

data/icons/hicolor/symbolic/apps/app.driftwood.Driftwood-symbolic.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="15 33 70 34">
+  <path fill="none" stroke="#241f31" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" stroke-miterlimit="10" d="M52.104,56.311c0-3.775,3.061-6.836,6.836-6.836c3.777,0,6.836,3.061,6.836,6.836 M62.293,55.785c0-1.652-1.34-2.99-2.994-2.99c-1.65,0-2.988,1.338-2.988,2.99 M48.948,56.311h19.983 M59.789,46.319l-13.996-1.577c-3.155-1.053-4.207-7.363-4.207-7.363l-3.154,2.104c0,0,2.103,4.207-1.053,4.207l-5.784-0.525c-3.194,0-5.784,2.59-5.784,5.785l23.137,7.361c0-5.519,4.474-9.991,9.991-9.991c5.52,0,9.992,4.473,9.992,9.991 M45.793,50l-6.311-3.154l-9.465-1.053 M35.276,48.949L39.482,50 M37.379,59.466l10.518,3.155h10.517 M74.189,56.311H80.5l-8.414-3.155 M64.725,59.466H53.156 M31.068,57.362l-6.31-2.104 M19.5,50l18.932,6.311" />
+</svg>