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, 18 insertions, 9 deletions

diff --git a/src/main.c b/src/main.c
index 16e706b..70343d2 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1,4 +1,8 @@
+#include <locale.h>
+
 #include <adwaita.h>
+#include <glib/gi18n.h>
+
 #include "gsr-window.h"
 
 /* ── About dialog ────────────────────────────────────────────────── */
@@ -29,7 +33,7 @@ on_about_action(GSimpleAction *action,
     const char *developers[] = { "dec05eba", NULL };
     adw_about_dialog_set_developers(about, developers);
 
-    adw_about_dialog_add_credit_section(about, "Adwaita Port",
+    adw_about_dialog_add_credit_section(about, _("Adwaita Port"),
         (const char *[]){ "Trung Lê", NULL });
 
     adw_dialog_present(ADW_DIALOG(about), GTK_WIDGET(win));
@@ -52,25 +56,25 @@ on_shortcuts_action(GSimpleAction *action,
         adw_shortcuts_dialog_new());
 
     /* ── Stream section ─── */
-    AdwShortcutsSection *stream_sec = adw_shortcuts_section_new("Stream");
+    AdwShortcutsSection *stream_sec = adw_shortcuts_section_new(_("Stream"));
     adw_shortcuts_section_add(stream_sec,
-        adw_shortcuts_item_new("Start / Stop streaming", "<Alt>1"));
+        adw_shortcuts_item_new(_("Start / Stop streaming"), "<Alt>1"));
     adw_shortcuts_dialog_add(dialog, stream_sec);
 
     /* ── Record section ─── */
-    AdwShortcutsSection *record_sec = adw_shortcuts_section_new("Record");
+    AdwShortcutsSection *record_sec = adw_shortcuts_section_new(_("Record"));
     adw_shortcuts_section_add(record_sec,
-        adw_shortcuts_item_new("Start / Stop recording", "<Alt>1"));
+        adw_shortcuts_item_new(_("Start / Stop recording"), "<Alt>1"));
     adw_shortcuts_section_add(record_sec,
-        adw_shortcuts_item_new("Pause / Unpause recording", "<Alt>2"));
+        adw_shortcuts_item_new(_("Pause / Unpause recording"), "<Alt>2"));
     adw_shortcuts_dialog_add(dialog, record_sec);
 
     /* ── Replay section ─── */
-    AdwShortcutsSection *replay_sec = adw_shortcuts_section_new("Replay");
+    AdwShortcutsSection *replay_sec = adw_shortcuts_section_new(_("Replay"));
     adw_shortcuts_section_add(replay_sec,
-        adw_shortcuts_item_new("Start / Stop replay", "<Alt>1"));
+        adw_shortcuts_item_new(_("Start / Stop replay"), "<Alt>1"));
     adw_shortcuts_section_add(replay_sec,
-        adw_shortcuts_item_new("Save replay", "<Alt>2"));
+        adw_shortcuts_item_new(_("Save replay"), "<Alt>2"));
     adw_shortcuts_dialog_add(dialog, replay_sec);
 
     adw_dialog_present(ADW_DIALOG(dialog), GTK_WIDGET(win));
@@ -154,6 +158,11 @@ on_activate(GtkApplication *app, gpointer user_data)
 int
 main(int argc, char *argv[])
 {
+    setlocale(LC_ALL, "");
+    bindtextdomain(GETTEXT_PACKAGE, GSR_LOCALEDIR);
+    bind_textdomain_codeset(GETTEXT_PACKAGE, "UTF-8");
+    textdomain(GETTEXT_PACKAGE);
+
     g_autoptr(AdwApplication) app = adw_application_new(
         "com.dec05eba.gpu_screen_recorder",
         G_APPLICATION_DEFAULT_FLAGS);