Use a dedicated keybinding for hunk navigation in the main view

Previously the patch_explorer and merge_conflicts controllers reused
Universal.PrevBlock/NextBlock for moving between hunks (or conflicts) in the
main view, sharing keys with the global side-window cycle. The two operations
are conceptually distinct: cycling side windows is a global navigation gesture,
while next/prev hunk acts on the diff in the main view. Tying them together also
blocks adding <tab>/<backtab> as side- window-cycle keys, because <tab> already
means "toggle panel" in the staging view.

Add Main.PrevHunk/NextHunk to the existing KeybindingMainConfig (which already
groups bindings for the main view across staging, patch building, and merge
conflicts) and switch both controllers to it. The defaults match the active key
set those controllers had before (<left>/<right>/h/l), so the user-visible
behavior is unchanged.