feat: added chart appearance settings in panel

[ahrefabhi]

📄 Summary

This PR introduces a Chart Appearance configuration section in the panel settings, allowing users to customize how their charts looks. It adds support for fill modes in time series charts and refines panel layout and styles, including making the panel configuration area resizable, to improve overall usability and visual consistency.

Screenshots / Screen Recordings (if applicable)

Issues closed by this PR

Closes https://github.com/SigNoz/engineering-pod/issues/4205

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✅ Change Type

• ✨ Feature
• ♻️ Refactor
• 🛠️ Infra / Tooling (panel configuration UX)
• 🧪 Test-only (if tests were added/updated alongside behavior)

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🧪 Testing Strategy

• Tests added/updated:
  • Unit and/or integration tests around:
    • New chart appearance configuration component(s).
    • Persistence and application of time series fill mode settings.
  • Snapshot updates if applicable for panel configuration UI.
• Manual verification:
  • Verified that Chart Appearance section renders correctly in the panel settings.
  • Confirmed that changing appearance options (including time series fill modes) updates the rendered chart as expected.
  • Verified panel config resizes correctly and preserves layout without visual regressions.
• Edge cases covered:
  • Panels created before this change (backward compatibility with existing configurations).
  • Default appearance behavior when no explicit appearance settings are set.
  • Handling of invalid or missing appearance settings data.

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⚠️ Risk & Impact Assessment

• Blast radius:
  • Panel editor UI and any panels using time series charts.
  • Shared UI components/styles used by the new appearance section.
• Potential regressions:
  • Existing panel configurations unintentionally changing appearance if defaults are misconfigured.
  • Layout issues or overflow problems due to resizable panel config.
  • Visual inconsistencies across different chart types if styles are not fully aligned.
• Rollback plan:
  • Revert this PR to restore previous panel settings behavior and styling.
  • If only appearance options are problematic, feature-flag or guard rendering of the Chart Appearance section while keeping unrelated refactors intact.

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📝 Changelog

No changelog as we're not enabling this to the users right now.

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📋 Checklist

• Tests added or explicitly not required
• Manually tested
• Breaking changes documented (none expected; new feature + UX enhancements)
• Backward compatibility considered (existing panels continue to work with sensible defaults)

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[AshwinBhatkal]

LGTM. Small nit comment