PAINTROID-814 Add import tool

[Amit-Matth]

This PR completes and stabilizes the Import Tool, enabling refined external asset integration with smart scaling, auto-centering, and a multi-placement workflow.

PAINTROID-814

New Features and Enhancements

• Official Sticker Gallery: Integrated a full-screen WebView that loads the official Catrobat figures download page, allowing for direct asset imports.
• Auto-Alignment: Stickers are now automatically centered in the middle of the canvas when imported.
• Intelligent Scaling: Tiny stickers are scaled up to a minimum interactable size (100px), and oversized stickers are scaled down to fit 80% of the canvas.
• Sticker Stamping Mode: The bounding box handles now remain visible after applying a sticker, allowing for multiple placements of the same asset in one session.

Refactorings and Bug Fixes

• Riverpod Service Migration: Encapsulated download and processing logic into a new StickerService and StickersGalleryProvider, moving away from local widget state.
• Eliminated Disposal Crashes: Fixed the "ref after disposal" crash by capturing providers and the Navigator instance before asynchronous gaps in the tool selection logic.
• Android Rendering Fixes: Enabled Hybrid Composition for the WebView to resolve flickering and the issue where content was hidden until scrolling.
• Undo/Redo Stability:
  • Created a dedicated ImportCommand to ensure undoing a sticker placement doesn't incorrectly switch the tool to the Clipboard tool.
  • Restored the refresh signal in the project loader to ensure Undo/Redo buttons update immediately after loading a file.
• Network & Compatibility: Added an SSL bypass for sticker downloads to support emulators with out-of-sync clocks and added missing INTERNET permissions to the manifest.
• Cleanup & Maintenance: Removed the accidentally committed hex_dump.ps1 script and updated the test suite to support the new ImportTool API.

Checklist

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