Separate CSV column selection into collapsible card

[rodrigogiraoserrao]

Motivation

• Improve the CSV Row Viewer UI by separating column selection from row browsing so the controls are clearer and less crowded.
• Make the column selector collapsible and collapsed by default to reduce visual noise for users who only want to view rows.
• Display column checkboxes in a single vertical list with the checkbox on the left for clear, accessible selection.

Description

• Moved the column selection UI out of the #viewer card into a new #column-selector card that contains a <details> block with a summary titled "Column selection" so it is collapsed by default.
• Removed the inline column-filter markup from #viewer and added #column-filter-card and #column-checkboxes inside the new #column-selector markup.
• Adjusted CSS to make the checkbox list vertical (.column-checkboxes { grid-template-columns: 1fr; }) and added .column-filter-details>summary styles to indicate the collapsible header.
• Updated JavaScript to reference the new columnSelector element and show/hide columnSelector together with columnFilterCard during CSV load and on parse errors, preserving existing checkbox and "select all" behavior.

Testing

• Ran git commit to record the change and it completed successfully.
• Verified git status --short shows a clean working tree after the commit.
• No automated UI test suite is configured for this static HTML tool, so no automated UI tests were run.

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Codex Task