Fix T3 Sidebar long child-menu scrolling

[Willhong]

What was wrong

T3 Sidebar rendered every child in an uncapped menu and an ordinary ul. With enough children, the menu extended beyond the thread viewport, but the list had no scroll range, so lower children were unreachable with both a desktop wheel and compact touch input. Fixes #2105. Standalone counterpart: SawyerHood/bb-plugin-t3sidebar#4.

What changed

• Cap the child menu at the smaller of 32rem and the available dynamic viewport height.
• Make the menu a flex column and the child list its min-h-0, overflow-y-auto scroll region.
• Contain scroll chaining and preserve mobile WebKit touch/momentum scrolling.
• Add a 26-child regression test that pins the bounded-menu and scroll-container contract.
• No host-daemon wire, public plugin API, CLI, configuration, guide, or documentation changes.

How you verified

• pnpm exec turbo run test typecheck --filter=bb-plugin-t3sidebar — 81 tests passed in the scroll-only iteration; typecheck passed.
• Desktop Chrome at 1728 × 813, 26 children:
  • before: menu 1,157px; list maxScroll 0; wheel 500px left scrollTop = 0
  • after: menu 512px; list maxScroll 645px; wheel 500px moved scrollTop = 500
• The regression test fails on the old classes and passes with this patch.

Fixes #2105

AGENT GENERATED: by GPT-5.6

[Willhong]

Standalone implementation counterpart: SawyerHood/bb-plugin-t3sidebar#6