refactor(ui): unify dropdown action rows

[beruro]

Summary

• extend shared DropdownItem with action-menu role, width, focus, label, and submenu-state props
• centralize Enter/Space activation and disabled tab-order behavior
• migrate Session actions, GitHub issue actions, Git sync actions, and terminal profile actions from duplicated raw rows
• preserve develop-only transcript-action gating while porting the Session menu
• repair the existing server-rendered GitHub Work Item test runtime import

Validation

• focused Vitest suites: 8 passed
• pnpm exec tsc --noEmit --pretty false
• Prettier and ESLint on all changed TypeScript/TSX files
• git diff --check

Frontend UI audit

6 fixes applied, 3 patterns kept with documented reasons, 0 new abstraction candidates. A systematic follow-up candidate records 23 remaining menuActionItem sites; they include submenu/control variants and were not converted site-by-site in this PR. Full report: docs/frontend-ui-audit-2026-07-24/DropdownActionRows.md.

Architecture audit

Covered compilation, shared interaction ownership, role/selection semantic separation, disabled/error parity, and caller migration. No persistence, wire protocol, resolver, or background lifecycle behavior changes.

[beruro]

Restacked onto #512. This PR now contains only the GitHub work-item dropdown migration and focused coverage/audit delta (3 files). Merge #512 first, then retarget to develop for CI.