Dashboard: burn-rate gauge (speedometer) widget

[tangentlin]

Motivation

The dashboard (src/dashboard/) currently shows historical series (cost over time, by model, by agent) via Chart.js. A live-feeling "burn rate" gauge — a speedometer/fuel-gauge needle showing current tokens/min or $/hr — would be the dashboard's signature visual and directly on-brand for "Burnbar."

Proposal

Add a gauge widget (needle + arc, animated transitions between readings) showing the current burn rate, with color bands (cool → hot) as intensity rises.

Feasibility notes

• The dashboard is a full BrowserWindow with Chart.js already bundled via esbuild — gauges are achievable with a Chart.js gauge/doughnut-based plugin or hand-rolled canvas, no new architectural surface needed.
• Requires the same burn rate derivation as the tray-flame idea (issue: tray icon animated flame) — worth deriving it once in derive.ts/capture-service.ts and sharing between the tray and dashboard rather than computing it twice.
• Read-only via the existing archive:get-series IPC channel; if burn rate needs finer-grained (sub-daily) data than the current archive grain, that's a scoping question to resolve before implementation.

Acceptance criteria

• Gauge widget added to the dashboard, animating needle transitions between readings rather than snapping.
• Burn-rate metric is derived once and shared with any other consumer (e.g. the tray flame animation) rather than duplicated.
• Degrades gracefully (e.g. needle at zero / "no recent activity") when there's no recent usage.