[Refactor] Extract useFocusTrap hook from DrawerLayout and AgentSelector

[samzong]

Summary

Extract duplicated focus-trap logic from DrawerLayout and AgentSelector into a single useFocusTrap hook. Eliminates ~40 lines of duplicated keyboard/focus management code and fixes an inconsistency where DrawerLayout did not properly save/restore previous focus.

Type of change

• [Refactor] internal cleanup

Why is this needed?

Both DrawerLayout and AgentSelector contained nearly identical inline focus-trap implementations: Tab wrapping, Escape handling, save/restore of previous focus, and initial focus targeting. The duplication violated DRY and the two implementations had subtle inconsistencies — DrawerLayout relied on a hardcoded menuButtonRef for focus restoration instead of saving document.activeElement on open, which meant edge-swipe opens could lose focus context.

What changed?

• New packages/pwa/src/hooks/useFocusTrap.ts — reusable hook with signature useFocusTrap(containerRef, open, onClose, initialFocusSelector?)
• DrawerLayout.tsx — removed 38-line inline focus-trap useEffect, removed menuButtonRef, simplified closeDrawer to one line, replaced with single useFocusTrap call
• AgentSelector.tsx — removed handleKeyDown useCallback (26 lines), focus-management useEffect (14 lines), and prevFocusRef; replaced with single useFocusTrap call using initialFocusSelector to skip the close button

Architecture impact

• Owning layer: renderer
• Cross-layer impact: none
• Invariants touched from docs/architecture-invariants.md: none
• Why those invariants remain protected: change is renderer-only, no state management or persistence logic affected

Linked issues

Closes #

Validation

• pnpm check (typecheck + lint + format + dead-code)
• pnpm build
• Manual smoke test
• Not run

Commands, screenshots, or notes:

pnpm check passes (only pre-existing todo.md format warning)
tsc --noEmit on useFocusTrap.ts passes in isolation

Screenshots or recordings

No visual changes — behavior is identical.

Release note

• No user-facing change. Release note is NONE.
• User-facing change. Release note is included below.

NONE

Checklist

• The PR title uses at least one approved prefix: [Feat], [Fix], [UI], [Docs], [Refactor], [Build], or [Chore]
• The summary explains both what changed and why
• Validation reflects the commands actually run for this PR
• Architecture impact is described and references any touched invariants
• Cross-layer changes are explicitly justified
• The release note block is accurate

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