Backend changes:
- Fix ListAllUserStats to calculate and return tag statistics
- Previously only returned name and timestamps, missing TagCount
- Now properly aggregates tags, pinned memos, and memo type stats

Frontend changes:
- Initialize user stats on app startup to populate tag data
- Show all tags when typing just '#' (fix empty Fuse.js search)
- Auto-refresh stats after creating/updating/deleting memos
- Fix Checkbox component ref warning with forwardRef

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