Update viewport effects to use dynamic visible dimensions.
viewport_effects() now takes visible_rows and visible_cols parameters
instead of hardcoded 20/8 values.
Scrolling logic uses these parameters:
- row_offset updates when nr >= row_offset + visible_rows
- col_offset updates when nc >= col_offset + visible_cols
Default registry initialized with visible_rows=20, visible_cols=8
for MoveSelection, MovePanelCursor, and other commands.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)
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