feat: snapshot dashboard

[sunchanglong]

Adds Snapshot Dashboard — a point-in-time, self-contained copy of a Perses dashboard that can be saved or exported and opened later without live data sources.

• Export from live dashboard — Capture the current time range, variable values, and panel query results into spec.snapshot on the dashboard JSON.
• Frozen view — Snapshot dashboards render from embedded data only; no GreptimeDB/Prometheus queries run on open.
• Preserved layout — Panel layout, chart types, and titles stay the same as the source dashboard.
• Read-only filters — Top variables and time range reflect values at export time and cannot be changed or refreshed.
• Save or download — Export via “Save as Snapshot Dashboard”: persist to the dashboard list or download a JSON file.
• Import & open — Snapshot dashboards are detected on import and open in dedicated snapshot view mode.n