feat: add gettext i18n with translations (vi, de, fr, it, es, pt)

Wire up internationalization for the app and ship six translations.

Infrastructure:
- meson: import i18n, define GETTEXT_PACKAGE/GSR_LOCALEDIR, add po/ subdir,
  and translate the .desktop and AppStream metainfo via i18n.merge_file
  (renamed to .desktop.in / .metainfo.xml.in).
- main.c: setlocale + bindtextdomain/textdomain so the catalog is loaded.
- po/: LINGUAS, POTFILES.in, meson.build (glib preset).

String marking:
- Wrap every user-visible string with _() across main.c, gsr-window.c, the four
  pages and the shortcut dialog (titles, subtitles, labels, buttons, combo
  display names, notifications, toasts, dialogs, menu items).
- Leave ids/keys untranslated: codec/container/service ids, GAction/icon/CSS
  names, GVariant/format strings, accelerators, etc.

Linting:
- Re-enable gobject-linter's untranslated_string rule (now that i18n exists) as
  an error so new user-visible strings must be wrapped.

Translations (141 messages each, %s/%d/markup placeholders preserved):
- Vietnamese (vi), German (de), French (fr), Italian (it), Spanish (es),
  Portuguese (pt). Codec names, "OK", proper nouns and the product name are
  intentionally left in English. Verified: msgfmt --check-format passes and the
  compiled catalogs resolve (e.g. "Record" -> "Ghi hình" / "Aufnahme" /
  "Enregistrement").

1 files changed, 10 insertions, 0 deletions

diff --git a/com.dec05eba.gpu_screen_recorder.desktop.in b/com.dec05eba.gpu_screen_recorder.desktop.in
new file mode 100644
index 0000000..53226ae
--- /dev/null
+++ b/com.dec05eba.gpu_screen_recorder.desktop.in
@@ -0,0 +1,10 @@
+[Desktop Entry]
+Type=Application
+Name=GPU Screen Recorder
+GenericName=Screen recorder
+Comment=A gpu based screen recorder / streaming program
+Icon=com.dec05eba.gpu_screen_recorder
+Exec=gpu-screen-recorder-adw
+Terminal=false
+Keywords=gpu-screen-recorder;screen recorder;streaming;twitch;replay;shadowplay;
+Categories=AudioVideo;Recorder;