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)

apps.json

@@ -7,9 +7,9 @@
       "name": "SpotiFLAC Mobile",
       "bundleIdentifier": "com.zarzet.spotiflac",
       "developerName": "zarzet",
-      "version": "4.5.0",
+      "version": "4.5.1",
       "versionDate": "2026-05-06",
-      "downloadURL": "https://github.com/zarzet/SpotiFLAC-Mobile/releases/download/v4.5.0/SpotiFLAC-v4.5.0-ios-unsigned.ipa",
+      "downloadURL": "https://github.com/zarzet/SpotiFLAC-Mobile/releases/download/v4.5.1/SpotiFLAC-v4.5.1-ios-unsigned.ipa",
       "localizedDescription": "SpotiFLAC Mobile is written in Flutter. Download tracks in true FLAC from Tidal, Qobuz, & Amazon Music.",
       "iconURL": "https://raw.githubusercontent.com/zarzet/SpotiFLAC-Mobile/main/assets/images/logo.png",
       "size": 37191956