- Remove duplicate mobile-close-btn from HTML
- Remove dead .mobile-close-btn CSS rules; unhide .close-preview at all viewports
- Change btnClearPreview tooltip from 'Hide workspace panel' to 'Close'
- Update tests across test_sprint41.py, test_sprint44.py, test_issue781.py,
and test_mobile_layout.py to match new single-button model
At the @media(max-width:900px) breakpoint both .close-preview and .mobile-close-btn were visible simultaneously. Since boot.js wires both to handleWorkspaceClose(), only the mobile-close-btn needs to show at that width. Adds .close-preview{display:none} to the 900px media block.
Fixes#781