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.
