PAINTROID-761-4: Integrate legacy image parser to app and handle fallback cases

[Amit-Matth]

This Pull Request corresponds to Stage 4 of our backward compatibility integration. It focuses on connecting the legacy model parsing engine to the application's file loader, ensuring robust fallback recovery for corrupted or truncated files, and aligning the deserialization with the Kryo 5.5.0 specification.

PAINTROID-761

New Features and Enhancements

• App File Loader Integration: Refactored LoadImageFromFileManager to explicitly support .catrobat and .bin (legacy catrobat-image) file extensions, routing them directly through the CatrobatImage.fromBytes parsing engine.
• Kryo 5 String Deserialization Specification Alignment: Aligned the string parser to fully conform to the Kryo 5.5.0 specification. Implemented optimized ASCII string decoding (checking for the MSB = 1 flag on the last byte) and UTF-8 VarInt flag-prefixed decoding. Integrated malformed-allowed UTF-8 decoding to prevent character encoding crashes.
• Commands List Size Deserialization Fix: Corrected a critical bug in legacy_model_parser.dart where the commands list size was incorrectly parsed using variable length checks, causing parsing errors on files with $\ge 11$ commands. The list size is now correctly read directly as a 4-byte int32 matching native Paintroid's serializer format.
• LoadCommand Deserialization & Transformation: Added support for decoding LoadCommand (class ID 20) and transforming it into a modern ClipboardCommand at origin Offset.zero with scale 1.0.
• Toast Alert for Unsupported Tools: Resolved a scoping bug in CatrobatImage.fromBytes where the hasUnsupportedCommands flag value was lost. Users are now correctly notified via a Toast warning when unsupported legacy operations (such as the Smudge tool) are safely bypassed.
• Undo/Redo Initialization Fix: Resolved an issue where Undo/Redo actions were disabled upon loading a legacy drawing on start-up. Added appBarProvider state updates in loadFromFiles and _loadFromPhotos inside io_handler.dart to trigger UI updates immediately when commands are loaded.
• Truncated File Safeguard: Added a !reader.hasRemaining safeguard in legacy_model_parser.dart to prevent app crashes when loading .catrobat-image files that were corrupted or cut off early by the legacy Android app. The parser now gracefully stops and recovers whatever strokes were successfully saved before the EOF.
• Fixed Collection Size Decoding: Fixed a byte misalignment issue where Kryo used 1-byte VarInts for generic object sizes, but Paintroid wrote explicit 4-byte Int32s for path actions and colors. Updated the parser to properly consume absolute 4-byte integers for these collections.

Refactorings and Bug Fixes

• Removed all debug print statements from the codebase to align with strict clean coding and contributing guidelines.
• Added missing trailing commas and fixed unused local variables (such as matchedHeader in catrobat_image.dart) to ensure the codebase is free of compiler or static analysis warnings.
• Updated unit test vectors in kryo_reader_test.dart to match correct Kryo 5.5.0 string specification formats.

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