Optimizes React Query migration with performance and consistency improvements:

Performance:
- Memoize AuthContext and InstanceContext provider values to prevent unnecessary re-renders
- Convert InstanceContext getter functions to useMemo hooks
- Fix refetchSettings to avoid state dependency that caused frequent recreations

Type Safety:
- Replace 'any' types in useAttachmentQueries with proper protobuf types
- Add Attachment and ListAttachmentsRequest type imports

Query Key Consistency:
- Replace hardcoded ["users", "stats"] with userKeys.stats() factory function
- Ensures consistent cache key management across mutations

Developer Experience:
- Rename unused useCurrentUser to useCurrentUserQuery to avoid confusion
- Add documentation explaining AuthContext-based vs React Query current user hooks
- Update internal references in useNotifications and useTagCounts

All changes verified with TypeScript compilation and build tests.

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>