Demo DB: feature-rich seed (12 objects, 500-row audit log) + regenerate action in Settings

- Expand demo schema to exercise every SQLite-available feature: composite PK
  (order_items), self-referencing FK (categories), JSON columns (lowercase
  'json' so the cell popover triggers), BLOBs (files), no-PK table (page_views,
  rowid editing), TEXT PK (app_settings), empty table (marketing_campaigns),
  read-only view (order_summary), CHECK/UNIQUE/defaults/indexes, smart-sort
  tiers, and a 500-row audit_log for pagination/virtualization/filter demos
- Stamp PRAGMA user_version = 2; stale demo files auto-recreate on launch
  (schema is idempotent, guarded inserts)
- Add regenerate_demo_db command: drops live pool handle, wipes + re-seeds
  the demo file; new Settings > General > Demo 'Regenerate demo' button with
  confirm dialog, loading state, toasts
- Fix Re-add demo to use the app data dir (was temp dir) so both paths match
  startup; reload connections after re-add
- Add 9 unit tests (demo.test.rs) covering objects, seed counts, JSON
  validity, FK consistency, view queryability, idempotency, constraint
  shapes, and stale/regenerate file flows

src-tauri/src/lib.rs

@@ -123,6 +123,7 @@ pub fn run() {
             keychain::get_connection_ssh_passphrase,
             keychain::delete_connection_ssh_passphrase,
             demo::recreate_demo_db,
+            demo::regenerate_demo_db,
             backup::detect_pg_tools,
             backup::pg_dump,
             backup::pg_restore,