Closes the v0.4+ SQLDBM-parity roadmap.

- lib/shareBundle.js generates a self-contained HTML file (entity
  cards grouped by subject area, relationships table, legend, inline
  CSS, prefers-color-scheme aware). Zero external assets.
- ShareBundleDialog.jsx — Modal with title/description input,
  Download + Copy HTML, sandboxed-iframe preview. Pulls from the
  currently adapted + domain-filtered schema.
- Snapshots backed by git tags: new /api/git/tags GET / POST / DELETE
  endpoints with annotated-tag creation on HEAD, name validation
  mirroring git's rules.
- SnapshotsDialog.jsx lists tags with commit/date/subject, lets
  users create and delete. Name field auto-suggests the next
  vMAJOR.MINOR based on existing tags.
- Command palette gains a "Share" section with both entries —
  no new topbar real-estate needed.

Co-authored-by: duckcode2025 <kranthi@duckcode.ai>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>