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)

android/app/src/main/kotlin/com/example/temp_project/MainActivity.kt

@@ -1,5 +0,0 @@
-package com.example.temp_project
-
-import io.flutter.embedding.android.FlutterActivity
-
-class MainActivity : FlutterActivity()