SmartCost: style switches green, fix chart scale labels and add toggle test

[TylonHH]

Motivation

• Make the smart-cost enable switches visually indicate the enabled state with the corporate green color.
• Fix the chart scale control by removing a duplicated toggle and standardizing the labels to match the UI wording.
• Ensure the smart-cost UI works reliably for dynamic price/CO₂ scenarios and is covered by a unit test.

Description

• Add a smart-tariff-switch CSS class to the active and relative switch inputs and set :checked to use var(--evcc-green) for background and border.
• Remove the duplicate chart scale button group and keep a single, controlled toggle driven by scaleMode in SmartTariffBase.vue.
• Update English labels in i18n/en.json to "Min–Max" and "0–Max" for the chart scale strings.
• Add a unit test assets/js/components/Tariff/SmartTariffBase.test.ts that mocks $t/$i18n and verifies the chart scale buttons toggle correctly.

Testing

• Ran npm test -- SmartTariffBase.test.ts and the unit test completed successfully (pass).

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Codex Task