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/update.sh

@@ -0,0 +1,36 @@
+#!/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