#453 Allow downloading multiple selected interview reports

Let candidates save multiple attempts without repe## Summary

• Add checkboxes to past interview attempts, selected by default.
• Add Select all / Deselect all and Download selected controls.
• Download only selected reports visible under the current filters.
• Preserve selections when filters change and reset them when history is reloaded.
• Reuse the existing Markdown download logic to preserve filenames and report content.

Screenshots

Before

After — default state

After — after clicking batch download

After — filtered with no matching reports

Testing

• Manually verified selection, filtering, and multiple Markdown downloads in Chrome on Windows.
• Passed ESLint, Prettier, DOM contract tests, and download unit tests.
• Added browser tests for selection and batch downloads; these are pending CI execution.

Closes #444titive downloads.

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Summary by cubic

Adds checkboxes to past interview attempts so you can download multiple saved reports at once instead of one by one.

• Attempts are ticked by default; the Select all / Deselect all and Download selected controls sit above the list.
• Batch downloads only the ticked reports visible under the current filters, reusing the existing per-report Markdown download.
• Selections persist across filter changes and reset when the history reloads.
• Browsers may block downloads after the first; the page tells users how to allow them.

Closes #444.