fix(repo-appearance): give shelf spine sections more breathing room
The rotated branch title was crowded against the first tab slot
(only 3pt gap) and the last tab slot was crowded against the
bottom-controls divider (0pt gap), so the spine read as one
continuous strip instead of three sections.
Introduce a `sectionGap` metric (10pt) for major spine boundaries:
- header → tab list: tabListContent's `.padding(.top)` switches
from `slotSpacing` (3) to `sectionGap` (10).
- tab list → bottom controls: bottomControls gets a new
`.padding(.top, sectionGap)` so the divider sits 10pt below the
last tab; the existing 3pt below the divider before the `+`
button stays the same.
`slotSpacing` (3pt between adjacent tabs) is unchanged — it still
tightly groups slots within the tab list.