feat: conditional formatting for pivot tables

[nishantmonu51]

• Adds conditional formatting to pivot tables so users can color measure cells based on value rules.
• Supported in both Explore (PivotToolbar / PivotFormattingPicker) and Canvas (inspector PivotConditionalFormatInput) dashboards.
• Rules persist through proto state (dashboard.proto, fromProto/toProto) and are applied during column definition rendering.
• Includes unit tests in pivot-conditional-formatting.spec.ts.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

---

Developed in collaboration with Claude Code