feat(core,gui): phase 1 shell — per-page headers, sidebar footer, Preferences dialog

Restructure the app shell per docs/UI_UX_PLAN.md phase 1:

- Every destination page gets its own AdwHeaderBar via page_frame(), with
  page actions packed into it and a busy spinner beside the title.
- The sidebar is regrouped (files, then a Sync section) with a primary menu
  (Preferences, Keyboard Shortcuts, About) and a footer showing sync state,
  storage quota and the account with Sign Out.
- The Settings page is gone: startup, folder location, appearance and cache
  options move into an AdwPreferencesDialog (Ctrl+,); transfers move to the
  Sync page; developer details move into About's debug info.
- Inline CSS moves to a gresource stylesheet using libadwaita colour
  variables. The system accent is the default; the Proton purple accent is
  an opt-in preference (new `proton_accent` config key, defaults off).
- Add a keyboard shortcuts dialog (Ctrl+?).

libadwaita requirement stays at 1.5, so Ubuntu 24.04 remains supported.

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

@@ -10,4 +10,10 @@
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-offline-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-online-only-symbolic.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
+       high contrast; the Proton purple accent is a separate, opt-in provider. -->
+  <gresource prefix="/de/nils/protondrivelinux">
+    <file>style.css</file>
+  </gresource>
 </gresources>