feat(core,gui): gettext translations and language preference

The app, the tray and the search prompt route every user-visible string
through GNU gettext in the `pdfs` text domain, with named placeholders and
plural forms. po/ holds the template, update and build scripts, and catalogs
for de, en_GB, es, fr, it, nl, pl and pt_BR. Preferences gains a Language row
that overrides the system language from the next start (config `language`).

Packages compile the catalogs into /usr/share/locale and the desktop files
carry translated descriptions and keywords. docs/TRANSLATING.md describes
the workflow.

packaging/aur/proton-drive-for-linux/PKGBUILD

@@ -9,7 +9,7 @@ url="https://github.com/narrrl/proton-drive-linux"
 license=('MIT')
 depends=('fuse3' 'gtk4' 'libadwaita' 'webkitgtk-6.0' 'dbus' 'gcc-libs' 'glibc'
          'hicolor-icon-theme')
-makedepends=('cargo')
+makedepends=('cargo' 'gettext')
 optdepends=('perl-image-exiftool: thumbnails for camera RAW files'
             'gnome-keyring: credential storage over the Secret Service API'
             'kwallet: credential storage over the Secret Service API'
@@ -68,6 +68,9 @@ package() {
   install -Dm644 packaging/proton-drive.service \
     "$pkgdir/usr/lib/systemd/user/proton-drive.service"
 
+  # Translations: compiled from po/ into /usr/share/locale.
+  po/build.sh "$pkgdir/usr/share/locale"
+
   install -Dm644 LICENSE "$pkgdir/usr/share/licenses/$pkgname/LICENSE"
   install -Dm644 README.md "$pkgdir/usr/share/doc/$pkgname/README.md"
 }