Add map import and export with durable refresh progress

docs/map-sharing.md

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+# Share a map with friends
+
+The original owner can choose **Export ZIP** in their map editor or on the public
+map page. The ZIP includes the saved map, selected panoramas, coordinates, and
+signed original-owner attribution. Save edits before exporting. Raw world files,
+server addresses, account IDs, tokens, and storage links are excluded.
+
+On **Servers**, choose **Import** beside search, then select the exported ZIP.
+Sign in first to keep the imported map in your dashboard. The ZIP limit is 256 MB.
+Files upload directly to private Supabase Storage, then the deployed image
+processor checks sizes, hashes, formats, and dimensions and creates the draft.
+Neither Minecraft nor the terrain-rendering worker is required for these imports.
+
+Imported maps are private and show **Owned by [original owner]**. The recipient
+can play solo or host an invite-only multiplayer room after processing, with no
+moderator approval. Submission, publication, and re-export of imported copies
+are blocked. The original owner keeps the normal export and public review flow.
+Private imported games do not contribute to the public leaderboard.
+
+Apply `supabase/migrations/20261007145000_map_sharing.sql` before deploying.
+Archive ownership signatures use a purpose-specific HMAC derived from the
+server's Supabase secret key. Rotating that key invalidates ownership signatures
+in older ZIPs; the original owner can export a new ZIP.
+
+Verification: `npm test`, `npm run typecheck -- --incremental false`,
+`node scripts/test-map-sharing-sql.mjs`, and `npm run build`.