Date snapshot banners from the snapshot and check captureSnapshot's poll URL #79

Three small faults on the share path remain after #77.

Snapshot banner date. A snapshot share's banner dates the snapshot from the share's createdAt, while the slot reads cite when each slot first served the frame it shows. A live share switched to a snapshot days after it was created shows the creation date.

Unchecked poll URL. captureSnapshot in src/lib/search/snapshot.ts joins the poll URL the API returns to its base without checking it, unlike the proxy routes, which build every upstream path through forwardablePath. This predates #77.

Corpus copy. src/content/dfo-docs/corpus/hub/build/share.md quotes the old snapshot dialog copy. The corpus is synced from upstream, so the fix belongs there.

Done when

• A snapshot banner shows the time the snapshot was taken.
• captureSnapshot refuses a poll URL that forwardablePath would refuse.
• The synced corpus quotes the current snapshot dialog.