feat: network storage section — panel probes mounted shares itself ...

Backend: shares + share_samples tables, background probe loop
(os.statvfs every GHARD_SHARE_INTERVAL, 7-day retention, ok=0 when
the mount is unreachable), CRUD + samples API, instant first sample
on create/update, shares in MCP panel_overview.

Frontend: /storage page — share cards with status badge/progress/
used-free-total, 24h fill chart, add/delete via modal + confirm.
ChartLine shows a visible point while there are fewer than 3 samples.

README: CIFS/NFS mount + fstab examples, GHARD_SHARE_INTERVAL row.

Co-Authored-By: Claude Code <noreply@anthropic.com>