feat(viz): format timestamp axes in the dashboard timezone — and reconcile the two timezone models

[xe-nvdk]

units.ts from #30 formats quantities. It does not format instants, and a time series axis is nothing but instants. That is missing on purpose — it needs a timezone, and Launchpad currently has two incompatible ideas of what a timezone is.

The conflict

• src/lib/dashboard/model.ts — timezone: 'utc' | 'browser' | <IANA name>, matching Grafana, which is what an imported dashboard carries.
• src/lib/stores/timezone.ts — 'utc' | 'local', a two-state toggle in the header, persisted as arc-timezone, and already wired through the logs and instance pages.

These are not the same type and the overlap is partial: 'browser' and 'local' mean the same thing under different names, and the store has no way to express America/Montevideo. A dashboard saved with an explicit IANA zone and then viewed through the existing toggle silently renders in the wrong one — off by hours, with no error.

Decide this before writing the formatter, not after. The recommendation is that the dashboard model wins, the store widens to 'utc' | 'browser' | string, 'local' migrates to 'browser' on read, and the header toggle becomes a picker on dashboard routes while staying a toggle elsewhere. But that is a call to make deliberately — it touches every existing consumer of the store.

The formatter

timeAxisFormatter(range: { from: number; to: number }, timezone: string): (ms: number) => string

• Tick granularity follows the span, not the tick count. A 5-minute window wants 15:04:05; a 3-day window wants Mar 12 15:00; a year wants Mar 2026. Rendering a full ISO string at every tick is how axes become unreadable.
• One format for the whole axis. Mixed granularity across ticks stops reading as a scale — the same rule formatAxisTicks already follows for quantities.
• Context goes in the axis label, not in every tick. The year appears once.
• Tooltips get full precision including the zone, since that is where someone copies a timestamp out of.

Traps

• Intl.DateTimeFormat with a timeZone option is the only correct way to do this. Adding an offset to a Date is wrong across a DST boundary, and a dashboard spanning one is exactly when someone is looking.
• Construct the Intl.DateTimeFormat once per axis, not per tick. It is one of the more expensive objects in the platform; building it inside a 60-tick loop on every redraw is measurable.
• An invalid IANA name from an imported dashboard makes Intl.DateTimeFormat throw, which takes the panel down. Validate at parse or catch at construction — do not let it reach a draw call.
• uPlot's default x unit is seconds unless opts.ms is set; frame.ts produces milliseconds. Whatever wires this needs to agree on one.

Scope

The reconciliation, src/lib/dashboard/time.ts + tests, and migrating the existing store consumers. Blocks the time series panel (#36) and the shared panel chrome (#34).

[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.