feat: Control Panel UI for trash, admin limits, dual-admin (Phase 5-6)

Phase 5 — Super-admin UI:
- Trash Management tab: list trashed items with days remaining,
  restore button, purge request (dual-approval), retention config
- Admin Limits tab (superadmin-only): set rule/CSV deletion limits,
  approval count, and limit config change limits per period
- Dual-admin approval integration in Approval Queue: approve/reject
  buttons auto-route to process_dual_approval for dual-admin requests
- Prompt modal helper for text input (retention, purge reason)
- is_superadmin flag in get_approval_queue response for UI gating

Phase 5 fix — Retention safety floor:
- Minimum trash retention set to 7 days (was 1)
- Prevents rogue super-admin from auto-purging within hours
- Backend and frontend both enforce 7-365 range

Phase 6 — Full test matrix (verified):
- RBAC: 28 role×action combinations tested
- Functional: trash lifecycle, admin limits, dual-admin approval
- Audit: all new action types logged with correct field attribution
- Edge cases: path traversal, XSS, NaN, overflow, negative values

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wildleo91