Replace deprecated localization APIs and add test coverage

Use AssetManifest.loadFromAssetBundle instead of manually parsing the
deprecated AssetManifest.json. Fix a broken localization key reference
(queue.cameraWithIndex → queue.itemWithIndex).

Replace the standalone scripts/validate_localizations.dart with proper
flutter tests (11 tests across two groups): file integrity checks
(directory exists, en.json present, valid JSON structure, language code
file names, deep key-completeness across all locales) and t() lookup
tests (nested resolution, missing-key fallback, parameter substitution,
partial-path fallback).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/settings/sections/queue_section.dart

@@ -112,7 +112,7 @@ class QueueSection extends StatelessWidget {
                               ? Colors.red 
                               : _getUploadModeColor(upload.uploadMode),
                         ),
-                        title: Text(locService.t('queue.cameraWithIndex', params: [(index + 1).toString()]) +
+                        title: Text(locService.t('queue.itemWithIndex', params: [(index + 1).toString()]) +
                             (upload.error ? locService.t('queue.error') : "") +
                             (upload.completing ? locService.t('queue.completing') : "")),
                         subtitle: Text(