Notebooks: migrate the Metrics tab into notebooks to unify dashboards (future, not urgent)

The Metrics tab (src/scenes/Editor/Metrics/, persisted as BufferType.METRICS with metricsViewState on the Buffer) is conceptually a sibling of notebooks: a grid of time-series charts persisted on a Buffer, with refresh. We want to fold Metrics into Notebooks so there's a single dashboard surface. Explicitly future / not urgent — don't block notebook work on it.

What Metrics is today

• Buffer carries metricsViewState (src/store/buffers.ts:50-57): global dateFrom/dateTo, refreshRate (AUTO/OFF/1s/5s/10s/30s/1m), sampleBy, viewMode, and metrics: Metric[].
• Each Metric is a predefined widget (not arbitrary SQL): metricType + tableId + color/position. The widget set lives in src/scenes/Editor/Metrics/widgets/ (WAL throughput/latency, row throughput, write amplification, avg txn size); each widget's getQuery() builds SQL from tableId + sampleBy + time range (src/scenes/Editor/Metrics/types.ts).
• Charts render with uPlot (createUplotOptions.ts, graph.tsx) under a global date-range picker, refresh rate, and sampleBy.

How that differs from notebooks

Notebooks use echarts (src/scenes/Editor/Notebook/CellChart/), arbitrary SQL per cell, and per-cell auto-refresh (no global date range / sampleBy); metrics are MCP-invisible while notebooks are MCP-controllable.

Prerequisite

We don't have a global (notebook-level) time-range or refresh rate today — notebooks only have per-cell auto-refresh, whereas Metrics is built entirely around global date-range + refresh + sampleBy controls. When these global controls are added (notebook-level time-range, and the notebook-level refresh rate from #577), the migration becomes feasible.

Migration shape

A metrics dashboard becomes a notebook of chart cells, the predefined widgets become SQL chart cells seeded from the widget query templates, and the global date-range / refresh / sampleBy become notebook-level controls. Needs a one-time data migration of persisted metricsViewState → notebookViewState (and BufferType.METRICS → NOTEBOOK).

Start with src/scenes/Editor/Metrics/, src/store/buffers.ts, and src/scenes/Editor/Metrics/types.ts, then compare the notebook cell and chart entry points under src/scenes/Editor/Notebook/CellChart/. Confirm the notebook-level controls from #577 before planning the migration. Done means Metrics dashboards can be represented as notebook chart cells, with persisted metrics state migrated to notebook state and the old buffer type retired.