#303 Let the candidate resize the test results panel

The fixed drawer can hide long compiler output during an interview. Allow candidates to trade editor space for results while keeping both areas usable and preserving the selected height.

Summary

• Add a vertical splitter between the editor and test results panel.
• Keep both panels usable by limiting the resizable range.
• Support keyboard resizing with arrow keys, Home, and End.
• Preserve the selected panel height across page reloads and window resizing.

Testing

• Added browser tests for dragging, keyboard controls, bounds, and saved height.
• All 8 related browser tests pass.
• Prettier checks pass for all changed files.
• Manually tested dragging, keyboard resizing, bounds, and saved height on Windows 11 with Google Chrome.

Closes #94

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Summary by cubic

Lets candidates resize the test results panel during an interview, so long compiler output no longer gets hidden by the fixed drawer. Adds a draggable splitter with keyboard support that preserves the selected height, while bounds keep both the editor and results area usable.

• The splitter supports arrow keys, Home, End, and Enter to reset, and stores the height in localStorage.
• Minimum size keeps the expanded "Add a case" form and results toggles visible; maximum size leaves a short, usable editor.
• The editor's old 25rem minimum height was removed so the drawer can reach its bounds without overflowing.
• The divider is hidden when execution is disabled.
• The drawer reclamps to fit when the window or surrounding editor content shrinks, without discarding the saved height, which is restored when space returns.