tui: add /diff slash command and move diff-mode toggle to Alt+G

The diff-mode cycle was bound to Alt+I, which is already consumed by the
info-widget toggle in handle_alt_key (it runs before navigation
shortcuts), making the diff toggle dead code in the local path. Move the
keybinding to the free Alt+G chord (works in both local and remote
paths) and add a discoverable /diff slash command that cycles or sets the
mode explicitly (off/inline/full/pinned/file/cycle/status).

Updates help overlay, info-widget tip, and the scroll-only key test to
match; adds unit tests for the new mode-name parser.