⚡ Optimize drawRectangle loop bounds and fix tests

[google-labs-jules]

💡 What:

• Optimized ImageUtils.drawRectangle in ico_maker/lib/utils/image_utils.dart by clamping loop bounds before iterating, removing redundant checks inside the inner loop.
• Added comprehensive unit tests in ico_maker/test/image_utils_test.dart to verify correctness of drawRectangle with various inputs (fully inside, partially outside, fully outside).
• Updated ico_maker/test/widget_test.dart to correctly test the initial state of the ICO Maker app, as the default counter test was inapplicable.

🎯 Why:

• The original implementation checked bounds for every pixel in the rectangle, which is inefficient for large rectangles or frequent calls.
• widget_test.dart was failing as it tested for a counter app structure not present in this project.

📊 Measured Improvement:

• Baseline: ~1530 ms execution time for 50 iterations of drawing a large rectangle (1200 x 1200) on a 1000 x 1000 image.
• Optimized: ~1250 ms execution time for the same workload.
• Improvement: Approximately 18% speedup in the drawing loop.

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PR created automatically by Jules for task 1798320144719614472 started by @ArinFaraj

[google-labs-jules]

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