fix(solar): suppress derived grid when meter inputs are missing

[gnacho]

Closes #135

Problem

With HAOS connected, unavailable consumption meters (Zigbee dropouts) collapse the derived consumption to 0. The bucket formula grid = consumption - production + batteryPower then fabricates impossible night exports (observed 26-Sep: -3.4 kW while the battery discharged ~3 kW and both consumption meters were down for ~10 min). The bogus buckets also inflated the day's grid-export KPIs.

Fix

• buildDaySeries: if any formula input is missing in a bucket (any consumption meter, or the battery power sensor), grid is suppressed (null) instead of computed with zeros. The feat(day-series): fill consumption gap from previous-day pattern when all meters drop #125 previous-day pattern fill still recomputes grid from the estimated consumption and marks the day estimated.
• computeLive: consumption meter down -> consumption null + warning alert; grid source entity down -> grid null (no night sign-flip or import fallback on fabricated zeros). Guards only apply while HA is connected.
• Frontend: PowerPoint.grid and LivePower.grid/consumption are nullable; fmtKw renders an em dash for null; day-KPI integration skips null grid buckets; LivePowerStrip and EnergyFlowDiagram show no-data with neutral flows.

Tests

• 4 new red-green tests reproducing the 26-Sep night scenario (full dropout, partial dropout, both live guards). Suite 115/115.
• tsc --noEmit + vite build clean.
• Deployed to the personal instance via the BYD overlay and validated.