fix(analytics): hint that an empty session list searched only the last 24 hours

[Topherhindman]

lk analytics session list under --experimental-auth printed only "No sessions found" whenever the last 24 hours were empty. With no --start, the Public API lists only the last 24 hours (its TimeRange default), so a project whose latest session was two days ago looked like it had none.

What changed

• An empty listing that left the window to the server gets a hint, as a status line after "No sessions found", worded like the paging hint (More results available — re-run with --cursor …):

  No sessions found
  Only the last 24 hours were searched — re-run with --start YYYY-MM-DD to look further back
  

• No hint when --start is set, when --cursor is (a cursor carries the window of the listing it came from), or with --json.
• The API-key listing is unchanged. --start's help is too: the flag is shared with the API-key endpoint, whose default window differs.

Testing

• TestSearchedDefaultWindow covers no --start, a --start, and a cursor.
• go test ./cmd/lk/ ./pkg/public/... and go vet pass.
• Against staging (--experimental-api-url https://beta-api.staging.livekit.cloud), on a project whose last session was 2026-10-07:
  • no --start prints the two lines above;
  • --start 2026-10-08 prints only No sessions found;
  • --json prints {"items": []} with no hint.