Dashboard fixes

[jayb]

Summary

This fixes a couple miscellaneous bugs/issues around dashboards:

1. Fixes an issue where configs copied from the Advanced section contain "__undefined__" strings as values
2. Standardizes height of some dropdown filter elements
3. Sets appropriate default name for Markup Variables

Testing Steps

1. Copy a config from the Advanced section. It should not contain "__undefined__" anywhere.
2. Open a chart with a filter and change the filter type between combobox/dropdown/multiselect/nested dropdown. They should all be 41px tall.
3. Add a Markup Variable to any visualization. It should automatically fill in the Variable Name field when you select a column.

[Copilot]

Pull request overview

This PR addresses several dashboard/editor issues by cleaning up Advanced Editor JSON output, standardizing filter control heights, and improving default behavior for Markup Variables.

Changes:

• Sanitizes Advanced Editor config data to remove "__undefined__" sentinel strings from copied/applied JSON.
• Tweaks styles for ComboBox/MultiSelect/NestedDropdown to align filter control heights.
• Adjusts Markup Variables Editor to auto-populate variable name/tag based on selected column/metadata key.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
packages/dashboard/src/components/DashboardFilters/DashboardFiltersEditor/components/FilterEditor.tsx	Avoids crash when markup-include visualizations have no contentEditor by using optional chaining.
packages/core/components/NestedDropdown/nesteddropdown.styles.css	Adjusts padding to help normalize nested dropdown height.
packages/core/components/MultiSelect/multiselect.styles.css	Updates border/font/padding and minor positioning tweaks to standardize MultiSelect appearance/height.
packages/core/components/ComboBox/combobox.styles.css	Adjusts input padding to help align ComboBox height with other filter controls.
packages/core/components/AdvancedEditor/AdvancedEditor.tsx	Adds sanitization to strip "__undefined__" sentinels and refactors config parsing into a helper.
packages/core/components/Footnotes/footnotes.css	Adds a footer class intended to control margin/padding for footnotes footer.
packages/core/components/Footnotes/Footnotes.tsx	Updates footer className handling (currently introduces a string interpolation bug).
packages/core/components/EditorPanel/components/MarkupVariablesEditor.tsx	Auto-sets variable name/tag from selected column/metadata (currently overwrites existing custom names).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.