chore: housekeeping cleanup and code deduplication

- Remove stray tracked files (root AndroidManifest.xml, build.gradle.bak, temp_project template)
- Move README-only images out of app asset bundle to reduce APK/IPA size (~1.68MB)
- Fix logo filename typo (transparant -> transparent)
- Deduplicate _readPositiveInt into shared int_utils.dart
- Deduplicate _themeModeFromString (reuse from theme_settings.dart)
- Remove deprecated LocalLibraryState.items getter
- Remove unused sqflite_common_ffi dependency
- Update apps.json version to 4.5.1
- Fix Flutter version in CONTRIBUTING.md (3.38.1 -> 3.41.5)
- Improve .gitignore patterns (NUL, *.bak, root AndroidManifest.xml)

lib/utils/int_utils.dart

@@ -0,0 +1,13 @@
+/// Parses a dynamic value to a positive integer (> 0), or returns null.
+///
+/// Accepts [num] and parseable [String] values.
+int? readPositiveInt(dynamic value) {
+  if (value == null) return null;
+  if (value is num) {
+    final asInt = value.toInt();
+    return asInt > 0 ? asInt : null;
+  }
+  final parsed = int.tryParse(value.toString());
+  if (parsed == null || parsed <= 0) return null;
+  return parsed;
+}