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, 20 insertions, 4 deletions

diff --git a/meson.build b/meson.build
index 0473058..f10f566 100644
--- a/meson.build
+++ b/meson.build
@@ -10,6 +10,10 @@ elif get_option('buildtype') == 'release'
     add_project_arguments('-DNDEBUG', language : 'c')
 endif
 
+i18n = import('i18n')
+gettext_package = meson.project_name()
+localedir = join_paths(get_option('prefix'), get_option('datadir'), 'locale')
+
 src = [
     'src/main.c',
     'src/gsr-window.c',
@@ -29,6 +33,8 @@ dep = [
 c_args = [
     '-DGSR_ICONS_PATH="' + join_paths(get_option('prefix'), get_option('datadir'), 'icons') + '"',
     '-DGSR_VERSION="' + meson.project_version() + '"',
+    '-DGETTEXT_PACKAGE="' + gettext_package + '"',
+    '-DGSR_LOCALEDIR="' + localedir + '"',
 ]
 
 if get_option('x11')
@@ -56,12 +62,22 @@ executable('gpu-screen-recorder-adw',
     c_args : c_args,
 )
 
-install_data(
-    files('com.dec05eba.gpu_screen_recorder.desktop'),
+subdir('po')
+
+i18n.merge_file(
+    input : 'com.dec05eba.gpu_screen_recorder.desktop.in',
+    output : 'com.dec05eba.gpu_screen_recorder.desktop',
+    type : 'desktop',
+    po_dir : 'po',
+    install : true,
     install_dir : join_paths(get_option('prefix'), get_option('datadir'), 'applications'),
 )
-install_data(
-    files('com.dec05eba.gpu_screen_recorder.metainfo.xml'),
+i18n.merge_file(
+    input : 'com.dec05eba.gpu_screen_recorder.metainfo.xml.in',
+    output : 'com.dec05eba.gpu_screen_recorder.metainfo.xml',
+    type : 'xml',
+    po_dir : 'po',
+    install : true,
     install_dir : join_paths(get_option('prefix'), get_option('datadir'), 'metainfo'),
 )
 install_subdir('icons/hicolor', install_dir : join_paths(get_option('prefix'), get_option('datadir'), 'icons'))