PAINTROID-733 Refactor IDeviceService to use Riverpod

[Amit-Matth]

This PR refactors IDeviceService to initialize the device size once at application startup and expose it via a synchronous Riverpod provider, avoiding repeated getSizeInPixels() calls and ensuring a stable device size throughout the app lifecycle.

PAINTROID-733

New Features and Enhancements

• Synchronous Size Provider: Migrated IDeviceService.sizeProvider from a FutureProvider to a synchronous Provider, eliminating the need for AsyncValue handling in the UI.
• Optimized Initialization: Updated main.dart to resolve the device size before app startup, ensuring the canvas and dependent services have valid dimensions from the first frame.

Refactorings and Bug Fixes

• Test Suite Recovery: Fixed UnimplementedError crashes across widget and integration tests by providing proper IDeviceService.sizeProvider overrides.
• Code Simplification: Removed redundant future-handling logic and .map() calls in LandingPage and DrawingCanvas, resulting in cleaner state management.
• Robust Integration Testing: Stabilized the heart shape test in shapes_tool_test.dart by making pixel assertions resilient to anti-aliasing variations.
• Consistent Test Environment: Updated manual ProviderContainer usage and adjusted canvas size expectations in landing_page_test.dart to match overridden test dimensions (1080×1920).

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