Separate and collapse CSV column selector card (#122)

### 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](https://chatgpt.com/codex/tasks/task_e_69c3c3f0a4f08325a4678927023530b4)

Author: rodrigogiraoserrao