Wave-5 refinement round 1 (owner's five fixes)

- New session: literal '+' glyph removed — one lucide PlusIcon (the
  duplicated glyph was PlusIcon + text '+').
- Top bar: a small pointer-events-none gradient under the header eases
  the scroller into the bar exactly like the dock's bottom fade.
- Option subtext (opt.description): .lh-prose — Source Sans 3; 'I don't
  know' copy stays JetBrains Mono.
- 'I don't know': now layout-matched with a radio circle; dashed border
  preserved as the deliberate non-answer.
- Separator spacing: options group pb-1 → pb-2 so the gap before the
  divider equals the 8px inter-option gap.

components/sidebar.tsx

@@ -248,7 +248,7 @@ function SessionsSidebar() {
 					className="mx-3 mb-1 flex items-center gap-1.5 rounded-md px-2 py-1.5 text-xs uppercase tracking-wider text-muted-foreground transition-colors hover:bg-accent hover:text-foreground"
 				>
 					<PlusIcon className="size-3" />
-					+ New session
+					New session
 					<span className="sr-only">create a new session</span>
 				</button>