History UI: improve tooltips, labels, stacks, units

[naltatis]

improves #29846

🔌 use Wh and kWh dependent on value (tooltip and y-axis)
📆 force tooltip time update on period change \cc @VolkerK62
👁️ hide labels for inactive entities in period
🎨 keep color assignment stable when switching between days
🫆 keep entity focus state when moving between time periods (if possible)

[sourcery-ai]

Hey - I've left some high level feedback:

• The maxVisibleKw/useWatts logic iterates over all visible series and slots on each recomputation; if this computed property is used frequently, consider deriving and caching the peak value once when building the chart data (e.g. alongside series/xAxis) to avoid repeated O(entities × slots) work.
• The paletteIndex ?? i pattern and related logic for deriving stable indices is repeated in several places (visibleSeries, colors, tooltip IDs, focus handling, legend items); consider extracting a small utility/helper to centralize this behavior and reduce the risk of future divergence.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `maxVisibleKw`/`useWatts` logic iterates over all visible series and slots on each recomputation; if this computed property is used frequently, consider deriving and caching the peak value once when building the chart data (e.g. alongside `series`/`xAxis`) to avoid repeated O(entities × slots) work.
- The `paletteIndex ?? i` pattern and related logic for deriving stable indices is repeated in several places (`visibleSeries`, colors, tooltip IDs, focus handling, legend items); consider extracting a small utility/helper to centralize this behavior and reduce the risk of future divergence.

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