Replaces separate jump-to-edge commands with a unified `JumpToEdge`
command. Simplifies the command registry using a macro and updates
`ScrollRows` to use a shared `viewport_effects` helper for consistent
scrolling behavior.
This fixes a bug where viewport scrolling was based on a hardcoded
constant (20) instead of the actual visible row count.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)
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