fix(umap): keep close button visible at small/medium window sizes

[lstein]

Summary

• At small (420px) and medium (500px) sizes the semantic-map titlebar buttons (~340px total) plus the album select (max-width 60%) overflowed the window's overflow: hidden right edge, clipping the close (×) button.
• Widen UMAP_SIZES.small 360→440 and .medium 440→520 (plot widths; window widths are plot+60).
• Tighten the titlebar icon-group flex gap from 8px to 3px, freeing ~30px across the 6 inter-button gaps.
• Size-detection thresholds in getCurrentWindowSize() still order correctly: small_window=500 < medium_threshold=520 ≤ medium_window=580 < big_threshold=800 ≤ big_window=860.

Test plan

• Open the semantic map and click the Small size icon — confirm the × close button is fully visible to the right of the controls icon.
• Click the Medium size icon — confirm the × close button is fully visible.
• Click Large and Fullscreen — confirm no regression (sizes/layout unchanged).
• Toggle shade (double-click titlebar) and restore — confirm the restored size is the previous unshaded size.
• Switch between albums via the titlebar dropdown with a long album name — confirm the close button still shows at small/medium.
• npm test (already verified locally: 339/339 passing).

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