refactor(monitoring): rebuild the Monitoring tab as a built-in dashboard

[xe-nvdk]

Context

src/lib/components/Monitoring.svelte is 398 lines of bespoke chart wiring: seven hand-built [number[], number[]] tuples, its own refresh timer that keeps polling while the tab is hidden, and no time range control beyond a duration dropdown. Once dashboards exist, it should be a dashboard.

This is the payoff issue — it proves the dashboard system is good enough to replace hand-written monitoring, and it deletes more code than it adds.

Depends on the dashboard view page and all panel types. Part of #19.

Scope

• A built-in dashboard definition shipped as JSON: memory, goroutines, HTTP requests and latency, query throughput and errors, storage reads and writes, GC, buffer, and compaction
• Built-ins are read-only but can be copied into an editable user dashboard ("Save as")
• The console's Monitoring tab renders this built-in dashboard instead of the bespoke component
• Delete Monitoring.svelte and MetricChart.svelte once nothing references them
• Where Arc exposes metrics only through /api/v1/metrics rather than as queryable tables, keep a small adapter so those panels still work

Where to look

• src/lib/components/Monitoring.svelte — transformChartData() enumerates exactly which metrics matter
• getTimeSeriesMetrics, getMetrics, getEndpointMetrics in src/lib/arcClient.ts
• MetricsSnapshot and EndpointMetrics in src/lib/arcClient.ts list every available field

Notes

Nothing should regress: every metric visible in the current Monitoring tab must still be visible, and it should gain time range, timezone, and zoom for free.

Acceptance criteria

• The built-in dashboard covers every metric the current tab shows
• Built-ins cannot be edited in place but can be copied
• Monitoring.svelte and MetricChart.svelte are deleted, not left orphaned
• The monitoring view gains drag-zoom and shared crosshair

[xe-nvdk]

Closing: the dashboards feature has been dropped from Launchpad. The implemented work was removed in #87 and this work will not be picked up. Recoverable from git tag pre-dashboards-removal-backup if the feature is ever revived.