fix: lock left sidebar row height on hover

Switch tree and heap rows from py-0.5 to a fixed h-[24px] + leading-none
so the hover-only kebab buttons can't stretch the row vertically as the
pointer crosses them.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/heaps/HeapsPanel.tsx

@@ -264,7 +264,7 @@ export function HeapsPanel() {
               <div
                 key={heap.id}
                 className={clsx(
-                  'group relative flex cursor-pointer items-center gap-1 rounded px-2 py-0.5 text-[12px]',
+                  'group relative flex h-[24px] cursor-pointer items-center gap-1 rounded px-2 text-[12px] leading-none',
                   isFiltered ? 'bg-primary/20 text-primary' : 'text-text hover:bg-surface-2',
                   // Active-heap row gets a soft primary wash so the
                   // user always knows where Pick / T will land, even