Inline tariff chart scale toggle and implement zero/range scaling with stepped bounds

[TylonHH]

Motivation

• Place the chart scale toggle inline with the energy price label/range so the control is visually adjacent to the price summary.
• Allow users to switch chart scaling between 0–max and min–max modes to avoid misleading visuals.
• Apply sensible rounding steps for axis bounds so min/max are clean and predictable for price and CO₂.

Description

• Move the chart scale toggle into the same line as the energy price label in SmartTariffBase.vue and keep it as a two-button btn-group.
• Add scaleMode data and setScaleMode method in SmartTariffBase.vue, compute chartScaleLabel, chartScaleZeroLabel, chartScaleRangeLabel, and chartScaleStep, and pass :scale-mode and :scale-step to TariffChart.
• In TariffChart.vue add scaleMode and scaleStep props, compute stepped min/range for range mode and 0-based bounds for zero mode, normalize and clamp bar heights in valueStyle, and add roundDown/roundUp helpers.
• Add translation keys for the new toggle labels in i18n/en.json and i18n/de.json.

Testing

• Started the dev server with npm run dev (served on port 7071) and it became ready, which succeeded.
• Ran a Playwright script to load the page and capture a screenshot, producing artifacts/tariff-chart-toggle-inline.png, which completed successfully.
• No unit or integration test suites were executed as part of this change.

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Codex Task