feat(cloud): link `cloud sessions dashboard` rows to per-session viewers

Add `--with-view` to `jcode cloud sessions dashboard`. When set, the
dashboard downloads each listed session and renders a local per-session
HTML viewer (via the Jade helper's `view --format html --output`), writing
them into a sibling `<dashboard-stem>-views/` directory and linking each
dashboard row's session id to its viewer with a relative href so the
generated site stays portable.

- Per-session viewer generation is best-effort: a failed `view` (e.g. a
  missing blob) logs a warning and leaves that row as plain text instead of
  aborting the whole dashboard. Progress is reported per session on stderr.
- New helpers: `dashboard_views_dir`, `sanitize_filename` (filesystem-safe
  session-id filenames), `relative_link` (dashboard-relative hrefs), and
  `generate_cloud_session_view_html`.
- `render_cloud_sessions_dashboard_html` now takes a session-id -> link map
  and only emits anchors for sessions that produced a viewer file.

Tests: `--with-view` arg parsing, linked vs unlinked row rendering,
filename sanitization, views-dir derivation, and relative-link computation.
Validated live against a fake helper exercising list+view, the per-session
failure/warning path, and the default (no --with-view) regression.