feat(dashboard): add battery endurance tracker widget

[jacobjurek]

Full-width gr26 BCU dashboard widget for monitoring the accumulator lap-by-lap during an endurance event.

Changes

• New "Battery Endurance Tracker" widget (gr26 → BCU, span 12), registered in registry.tsx.
• Manual lap capture (Lap +1) snapshots current live SOC / min cell voltage / max cell temp.
• Lap table with per-lap deltas and % change; latest row highlighted; stale-capture flagging.
• Projection cards: laps until max cell temp > 60 °C, SOC < 20 %, SOC = 0 %, from trailing per-lap rates (color-toned by urgency).
• Statistics: per-signal min/avg/max, avg lap time, and regression-projected SOC/V/T at the 35-min mark.
• Graph: 0–35 min dual-axis voltage/temperature line chart (data via /query/pairs) with dashed regression projection lines extended to each boundary, plus 60 °C / voltage-floor reference lines and a finish marker.
• Persistence: localStorage key battery_endurance_v1_<vehicleId>. Survives page refresh and Sentinel token expiry — the persisted table is decoupled from the live socket (uses useLiveSignals directly rather than wrapping LiveWidget), so it stays visible when the connection drops.
• Editable thresholds (temp max, SOC low, cell-V floor), persisted with the event.
• New dashboard/src/lib/batteryEndurance.ts (state/persistence + least-squares regression and projection math) with unit tests.

Verification

• npm run build, tsc --noEmit, eslint --max-warnings 0 clean; 14 unit tests pass (batteryEndurance.test.ts).
• Manually verified against the local stack: widget renders for GR26, lap capture + table persist across refresh and after clearing sentinel_access_token.