fix(shelf): show per-button tooltips on spine bottom controls
The whole spine had a `.help(book.displayName)` covering its full bounds,
so hovering the New Tab / Split buttons (or any tab slot) still showed
the book's branch name instead of the action label. Move the help to
the header where book identification belongs, and let each control
render its own tooltip — matching the format used by the horizontal
tab bar's trailing accessories ("Title (⌘…)") by pulling the
keybinding from `GhosttyShortcutManager`.