ess_billing: add projection window selector to burndown graph

[3kt]

Summary

• Adds a Projection window dropdown (7 days / 30 days) to the Vega burndown graph in the credits dashboard
• The dropdown controls how many days of recent billing data are used to compute the average daily burn rate for the "Expected consumption" projection line
• Addresses inaccurate runout predictions for serverless customers with tiered pricing, where short lookback windows overweight early-month high-tier costs

Test plan

• Import the credits dashboard locally
• Toggle the "Projection window" dropdown between 7d and 30d
• Verify the "Expected consumption" line updates accordingly
• Verify historical consumption and contractual target lines are unaffected
• Test with a serverless account that has tiered pricing to confirm 30d smooths out the early-month spike

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

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[elasticmachine]

💚 Build Succeeded

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• Commit: fe1c85f

History

• 💚 Build #40797 succeeded 3445e0f
• 💚 Build #40780 succeeded 27eeb82
• 💔 Build #40779 failed 3236f36