Fix items specifying height-only or width-only dimension having wrong proportions

[emosaru]

Summary

• Fixes Items specifying height instead of width has some display issues #4, Cascading margin issue? (Map getting misaligned with background item grid) #7
• IconDimensionMultiConverter now accepts three bindings: this dimension, the other dimension, and the image source. When only one dimension is specified, the missing dimension is derived from the image aspect ratio rather than falling back to the raw pixel size.
• Fixed the guard condition from > 0 to >= 0 so an explicit zero-dimension is honoured and doesn't fall into the proportional-scaling path.
• Added FallbackValue=False to BadgeText visibility binding to prevent flicker during binding initialisation.
• Changed Button template inner Grid background from Transparent to {x:Null} to avoid a background that unnecessarily consumes hit-test area.

Test plan

• Load CodeTracker, open Key Tracker variant
• Verify Prize Shuffle icon displays correctly (overlaid on the 2x2 dungeon grid)
• Verify Modes popout menu displays correctly
• Verify items that specify only h (height) render at correct proportions instead of being stretched to square
• Verify items that specify only w (width) still render correctly

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