feat(gui): branded app icon in the sidebar header and login

The sidebar header and the login page used the generic folder-remote-symbolic
icon. Ship the app icon from packaging/ in the GUI resources and use it in
both places, so the in-app brand matches the launcher even when the app runs
uninstalled.

crates/pdfs-gui/resources/pdfs.gresource.xml

@@ -9,6 +9,9 @@
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-upload-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-offline-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-online-only-symbolic.svg</file>
+    <!-- The app icon, from the one file packaging installs, so the in-app brand
+         matches the launcher and resolves even when nothing is installed. -->
+    <file alias="scalable/apps/io.narl.proton-drive-linux.svg" preprocess="xml-stripblanks">../../../packaging/io.narl.proton-drive-linux.svg</file>
   </gresource>
   <!-- The app stylesheet, loaded once at startup. Colours come from the
        libadwaita palette so the app follows the system accent, light/dark and

crates/pdfs-gui/resources/style.css

@@ -9,7 +9,6 @@
 
 /* Brand */
 .brand-title { font-size: 1.6rem; font-weight: 800; }
-.brand-icon { color: #6d4aff; }
 
 /* Sidebar footer */
 .sidebar-footer { padding: 6px 12px 12px 12px; }