fix(thread-list): prevent sticky header corner bleed-through

[kongenpei]

Summary

Sticky thread-list headers reveal scrolled row backgrounds through their rounded corners. Make the shared tier square; ordinary thread rows and action buttons keep their rounding.

 cn(baseClasses, "bg-sidebar", className,
+  "rounded-none",
 )

Fixes #5208.

Evidence

• Before: reproduced in Chrome using the actual SidebarStickyTier and app theme from current main. Label/project/parent tiers computed border-radius: 6px, exposing underlying row backgrounds at the corners.
  After: all three tiers compute 0px; corner pixels are covered, sticky positions are unchanged, and ordinary rows still compute 6px. Checked light/dark themes with normal, selected, open-in-split, and hover/menu-open states.
• pnpm exec turbo run test typecheck --filter=bb-plugin-thread-list: 37 files / 468 tests passed; typecheck passed.
• pnpm exec turbo run test --filter=@bb/app -- src/components/ui/theme.test.ts: 38 tests passed.
• pnpm exec turbo run lint --filter=bb-plugin-thread-list: 0 errors, 10 warnings.
• Independent code review found no blocking issues.

Merge Danger

Door: two-way — revert one class.

Blast Radius: cosmetic — shared header tiers are square, including before they stick. No changes to scrolling, sticky offsets, state backgrounds, or wire contracts.

AGENT GENERATED: by GPT-6