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>

pubspec.lock

@@ -106,7 +106,7 @@ packages:
     source: hosted
     version: "1.1.2"
   collection:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: collection
       sha256: "2f5709ae4d3d59dd8f7cd309b4e023046b57d8a6c82130785d2b0e5868084e76"
@@ -601,7 +601,7 @@ packages:
     source: hosted
     version: "1.1.0"
   path_provider:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: path_provider
       sha256: "50c5dd5b6e1aaf6fb3a78b33f6aa3afca52bf903a8a5298f53101fdaee55bbcd"