Keep T3 Sidebar child menus inside compact viewports

[Willhong]

What was wrong

T3 Sidebar unconditionally positioned its 20rem child menu with absolute right-0 relative to the header chip. Compact headers can place that chip farther left than the menu width, producing a negative left edge and clipping the menu outside the viewport. Fixes #2104. Standalone counterpart: SawyerHood/bb-plugin-t3sidebar#3.

What changed

• Keep desktop placement unchanged (absolute right-0).
• On compact viewports, preserve the 20rem maximum width and the trigger's preferred right alignment.
• Clamp the computed left coordinate only when the preferred position would cross the 8px viewport gutter.
• Recompute on window and visual-viewport resize/scroll so rotation and browser chrome changes stay aligned.
• Add regression coverage for the unclamped, clamped, and desktop branches.
• 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 — 83 tests passed; typecheck passed.
• Compact geometry at 390px:
  • trigger right 380px → preferred/aligned left 60px remains 60px
  • trigger right 250px → negative preferred left clamps to 8px
• The desktop regression keeps absolute right-0 w-80.
• iPhone 17 / iOS 26.4 Simulator Safari reproduced the pre-fix clipping.

Fixes #2104

AGENT GENERATED: by GPT-5.6

[Willhong]

Standalone implementation counterpart: SawyerHood/bb-plugin-t3sidebar#5