feat(sybil): add sybil detection admin UI and trust management pages (#61)

Frontend implementation for P2.10 sybil resistance. Adds admin dashboard
and management pages for the graph-based sybil detection system.

New pages:
- Sybil Detection dashboard: cluster list with filtering/sorting,
cluster detail with member enrichment, confirm dialogs for
dismiss/monitor/ban actions, behavioral flags section, trust graph
status card with manual recompute
- Trust Seeds management: seed list with manual/automatic badges,
add/remove dialogs, implicit seed protection

Modified pages:
- Admin Settings: PDS Provider Trust section with override management
(hostname + trust factor slider 0.0-1.0)
- Admin Layout: sidebar navigation links for new pages

Supporting changes:
- 13 new TypeScript types, 12 new API client functions
- 12 new MSW mock handlers with auth checks and CRUD
- All components pass vitest-axe (WCAG 2.2 AA)

29 new tests across 4 test files. All 552 tests pass.