Snapshot URL datasets locally on add; preview tabular data as a table

docs/manual-verification.md

@@ -44,6 +44,23 @@
 - [ ] Toasts stack, auto-dismiss, and fade as specified; the live-preview busy
       indicator appears for slow (>~1s) renders and clears after.
 
+## URL datasets (remote data snapshot)
+
+> Needs a real network, real CORS, and real offline — tests mock the fetch.
+
+- [ ] Add a dataset by URL from a CORS-friendly host (e.g. a GitHub raw `.csv` or a
+      vega-datasets URL) — Save shows "Fetching…", then the dataset appears profiled
+      (rows, columns, size) with a "Fetched <time>" line and the source address.
+- [ ] Go offline (DevTools → Network → Offline) and reload — a snippet that references
+      that URL dataset still renders from the local snapshot (no fetch at render time).
+- [ ] Add a URL that blocks cross-origin requests (or while offline) — the form shows a
+      cause-specific error and a "Paste data inline instead" button that switches to an
+      inline paste keeping the name/comment; no broken record is saved.
+- [ ] Refresh a URL dataset whose source changed — rows/size and the "Fetched" time
+      update; refreshing while offline raises an error toast and leaves the snapshot intact.
+- [ ] Build Chart from a fetched URL dataset works (columns are known); from an
+      unfetched URL reference the builder has no schema until Refresh.
+
 ## Visual sweep of the M6 surfaces
 
 - [ ] Library search / sort / empty states, the storage monitor, About & Donate