spike(mecatui): migrate help browser to the reusable scroll system

[kantord]

Implementation

Closes #1900

Plan / Interface PR: #2045
Approved baseline: 0b5bdb8a272744d9f24d0cf67ae9bf6e033f61f6

Completed acceptance contract

• Help uses a root-owned wrapped bounded.Viewport with stable responsive card width.
• Help retains its live-binding indicator, keyboard lifecycle, and compact fallback.
• Help owns wheel events while visible, including endpoints and non-overflow states, so hidden conversation state cannot move.
• The existing TUI guide documents responsive Help browsing.

Interfaces match approved contract: Yes.

No implementation deviation required an amendment.

Verification

go test ./cmd/mecatui/ui -run 'TestHelp(...)'
task docs
task test
task lint
task test:race
task site:build
go run ./cmd/mecademo