chore(l10n): add CI guard against ARB placeholder corruption

Adds tool/check_arb_placeholder_corruption.py, a static check that flags
literal "?" glued to an ICU placeholder brace in any lib/l10n/app_*.arb
value that has a placeholders metadata block. This is the exact corruption
shape fixed in the previous commit; flutter analyze doesn't catch it since
the JSON/ICU stays syntactically valid. Wired as a CI step before
flutter analyze so it fails fast.

Also audited lib/l10n/app_localizations_ext.dart (hand-maintained weekday/
month/date-sentence maps, not covered by ARB tooling): all 22 locale maps
have the full 13/13 keys with no corruption or leftover English — no
changes needed there.

.gitea/workflows/build.yml

@@ -27,6 +27,9 @@ jobs:
       - name: Obtener dependencias
         run: flutter pub get
 
+      - name: Verificar integridad de literales i18n
+        run: python3 tool/check_arb_placeholder_corruption.py
+
       - name: Analizar código
         run: flutter analyze --no-fatal-infos --no-fatal-warnings