Fix json display in leaderboards

[ericokuma]

Fixes APP-308: Display JSON in leaderboards and dimension tables, and enable filtering.

Previously, JSON dimension values were displayed as [object Object] and caused SQL errors (unsupported type map[string]interface {}) when filtered. This PR introduces a formatDimensionValue helper to properly stringify JSON, STRUCT, MAP, and ARRAY types. This ensures correct display in leaderboards and dimension tables, and allows filtering on these values without errors by passing stringified JSON to the backend.

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 (APP-308)
• 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!

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Linear Issue: APP-308

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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