Fix Rect.fromLTRB zero-dimension errors causing rendering crashes

[Copilot]

This PR fixes critical rendering pipeline crashes caused by zero-width/zero-height rectangles that are now invalid in newer Flutter/Dart versions.

Problem

The app was crashing during music rendering due to Rect.fromLTRB calls creating rectangles with zero dimensions. In Dart 3 and newer Flutter versions, these invalid rectangles throw errors instead of being silently accepted, causing the entire rendering pipeline to fail.

Root Causes Found

Two locations were creating problematic zero-dimension rectangles:

1. Music System Painter (lib/music_view/music_system_painter.dart:266):

   // BEFORE: Creates 0x0 rectangle
   magicOpacityFactor(Rect.fromLTRB(left, 0, left, 0))

2. Harmony Beat Renderer (lib/drawing/harmony_beat_renderer.dart:165):

   // BEFORE: Creates zero-width vertical line
   Rect.fromLTRB(bounds.left + leftOffset, bounds.top, bounds.left + leftOffset, bounds.bottom)

Solutions Applied

Fixed both issues by ensuring minimal valid dimensions while preserving the original visual intent:

1. Music System Painter:

   // AFTER: Creates minimal 1x1 rectangle for opacity calculation
   magicOpacityFactor(Rect.fromLTRB(left, 0, left + 1, 1))

2. Harmony Beat Renderer:

   // AFTER: Creates 1-pixel-wide vertical line
   Rect.fromLTRB(bounds.left + leftOffset, bounds.top, bounds.left + leftOffset + 1, bounds.bottom)

Verification

• Added comprehensive test coverage to prevent regression
• Thoroughly audited the entire codebase for similar patterns
• Confirmed fixes maintain original functionality while preventing crashes

The app should now run successfully with flutter run on web and iOS without Rect.fromLTRB errors, restoring proper music rendering functionality.

Fixes #11.

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