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.

po/LINGUAS

@@ -0,0 +1,8 @@
+de
+en_GB
+es
+fr
+it
+nl
+pl
+pt_BR

po/POTFILES.in

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+crates/pdfs-gui/src/activation.rs
+crates/pdfs-gui/src/app/main.rs
+crates/pdfs-gui/src/app/pages/activity.rs
+crates/pdfs-gui/src/app/pages/albums.rs
+crates/pdfs-gui/src/app/pages/browser.rs
+crates/pdfs-gui/src/app/pages/devices.rs
+crates/pdfs-gui/src/app/pages/locations.rs
+crates/pdfs-gui/src/app/pages/login.rs
+crates/pdfs-gui/src/app/pages/photo_viewer.rs
+crates/pdfs-gui/src/app/pages/photos.rs
+crates/pdfs-gui/src/app/pages/shared.rs
+crates/pdfs-gui/src/app/pages/shared_by_me.rs
+crates/pdfs-gui/src/app/pages/status.rs
+crates/pdfs-gui/src/app/pages/takeout.rs
+crates/pdfs-gui/src/app/pages/trash.rs
+crates/pdfs-gui/src/app/pages/verify.rs
+crates/pdfs-gui/src/app/widgets/details.rs
+crates/pdfs-gui/src/app/widgets/menu.rs
+crates/pdfs-gui/src/app/widgets/share_dialog.rs
+crates/pdfs-gui/src/app/widgets/thumbnails.rs
+crates/pdfs-gui/src/app/widgets/versions_dialog.rs
+crates/pdfs-gui/src/dmenu.rs
+crates/pdfs-gui/src/fzf.rs
+crates/pdfs-gui/src/i18n.rs
+crates/pdfs-gui/src/main.rs
+crates/pdfs-gui/src/prompt.rs
+crates/pdfs-gui/src/query.rs

po/build.sh

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+#!/usr/bin/env bash
+# Compile every catalog in po/LINGUAS into <localedir>/<lang>/LC_MESSAGES/pdfs.mo.
+# Packages pass their install tree, e.g. "$pkgdir/usr/share/locale"; for a run
+# from the source tree use target/locale with PDFS_LOCALEDIR=target/locale.
+set -euo pipefail
+
+if [ $# -ne 1 ]; then
+  echo "usage: $0 <localedir>" >&2
+  exit 2
+fi
+localedir="$1"
+po_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+while read -r lang; do
+  [ -n "$lang" ] || continue
+  mkdir -p "$localedir/$lang/LC_MESSAGES"
+  msgfmt --check --output-file="$localedir/$lang/LC_MESSAGES/pdfs.mo" "$po_dir/$lang.po"
+done < "$po_dir/LINGUAS"

po/update.sh

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+#!/usr/bin/env bash
+# Regenerate po/pdfs.pot from the sources listed in po/POTFILES.in, then merge
+# the new template into every catalog in po/LINGUAS. Run from anywhere.
+set -euo pipefail
+
+po_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "$po_dir/.."
+
+version="$(grep -m1 '^version' Cargo.toml | cut -d'"' -f2)"
+
+xgettext \
+  --language=Rust \
+  --from-code=UTF-8 \
+  --files-from=po/POTFILES.in \
+  --output=po/pdfs.pot \
+  --package-name=proton-drive-linux \
+  --package-version="$version" \
+  --msgid-bugs-address=https://github.com/narrrl/proton-drive-linux/issues \
+  --add-comments=Translators: \
+  --sort-by-file \
+  --keyword= \
+  --keyword=gettext \
+  --keyword=gettext_f \
+  --keyword=gettext_noop \
+  --keyword=pgettext:1c,2 \
+  --keyword=ngettext:1,2 \
+  --keyword=ngettext_f:1,2
+
+while read -r lang; do
+  [ -n "$lang" ] || continue
+  if [ -f "po/$lang.po" ]; then
+    msgmerge --quiet --update --backup=none --previous "po/$lang.po" po/pdfs.pot
+  else
+    msginit --no-translator --locale="$lang.UTF-8" --input=po/pdfs.pot --output-file="po/$lang.po"
+  fi
+done < po/LINGUAS