[flexdinesh]

• Activity rail width now equals --topbar-height (36px desktop, 44px coarse) instead of 48/52px, matching the thin header.
• Rail view/account buttons reuse header icon-button geometry: 32px items (44px touch), 16px icons, 1.7px strokes, --radius-control, same 2px inset. Previously 40px items, 20px icons, --radius-md.
• Coarse pointers add a 4px block inset so flush 44px items stay inside the island's rounded corners.
• DESIGN.md activity-bar rule updated to the new requirement.
• Browser test now asserts rail width == header height, rail item size/inset/radius == header Theme button, 16px rail icons.

Validation

• mise run check: pass (90 browser tests, Go, static, design checks).
• Manual: light/dark, Repos/Diff selected and hover; computed rail 36px, header 36px, items 32px, icons 16px.