[Refactor] Extract BottomSheet primitive from 3 PWA components

[samzong]

Summary

Extract the duplicated bottom-sheet pattern (backdrop + spring animation + drag-to-dismiss + drag handle + focus trap + escape key) from AgentSelector, GatewayDebugLog, and SettingsSheet into a single BottomSheet primitive.

Type of change

• [Refactor] internal cleanup

Why is this needed?

Three components copy-pasted ~40 lines of identical bottom-sheet infrastructure. The copies had silently diverged:

• GatewayDebugLog was missing role="dialog", aria-modal, aria-label, focus trap, and focus restore
• SettingsSheet was missing focus trap and focus restore
• All three lacked body scroll lock (background scrolls through the sheet on iOS Safari)

What changed?

• New: BottomSheet.tsx (~110 lines) — encapsulates backdrop, spring animation, drag-to-dismiss, drag handle, focus trap, focus restore, body scroll lock, escape key, safe area, and ARIA attributes
• Modified: AgentSelector.tsx (156 → 67 lines) — delegates sheet infrastructure to BottomSheet, keeps only agent-specific content
• Modified: GatewayDebugLog.tsx (192 → 132 lines) — delegates sheet infrastructure, gains role="dialog" / aria-modal / aria-label / focus trap / focus restore
• Modified: SettingsSheet.tsx (165 → 113 lines) — delegates sheet infrastructure, gains focus trap / focus restore
• Net: -64 lines, 3 accessibility bugs fixed, body scroll lock added

Architecture impact

• Owning layer: renderer (PWA)
• Cross-layer impact: none
• Invariants touched from docs/architecture-invariants.md: none
• Why those invariants remain protected: change is renderer-only, no protocol/state/persistence changes

Linked issues

Closes #

Validation

• pnpm lint
• pnpm test
• pnpm check:ui-contract
• pnpm check (full gate: lint + architecture + ui-contract + renderer-copy + i18n + dead-code + format + typecheck + test)
• Not run

Commands, screenshots, or notes:

pnpm check — all gates passed

Screenshots or recordings

No visual change — same animation, same thresholds, same styling. Behavioral fixes (focus trap, scroll lock) are not visible in screenshots.

Release note

• No user-facing change. Release note is NONE.

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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