Add toggle for tariff chart scaling (0–max vs min–max) with stepped range

[TylonHH]

Motivation

• Provide a user-controllable chart scale so bars can be shown from 0–max or from the observed min–max to avoid misleading visuals when values are tightly clustered.
• Round min/max to sensible steps for price and CO₂ so axis bounds are clean and predictable.
• Preserve a neutral fallback when data is empty or has zero range to avoid exaggerated heights.

Description

• Add scaleMode ("zero" | "range") and scaleStep props to TariffChart and wire them from SmartTariffBase via :scale-mode and :scale-step bindings.
• Implement min/max computation in valueInfo to support the two modes, using Math.min(0,min)/Math.max(0,max) for zero mode and stepped roundDown/roundUp for range mode.
• Normalize and clamp bar heights in valueStyle using (val - min) / range, and return a 50% neutral height when the range is 0 or value is invalid.
• Add UI toggle buttons in SmartTariffBase with translations and a chartScaleStep computed value that returns 1 for CO₂ and 0.01 for price.

Testing

• Started the dev server with npm run dev and the Vite server reported ready, which succeeded.
• Ran a Playwright script to load the page and capture a screenshot, producing artifacts/tariff-chart-scale-toggle.png, which completed successfully.
• No unit or integration test suites were executed for this change.

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Codex Task