feat(umap): album pulldown in semantic-map titlebar

[lstein]

Summary

• Convert the album-name label at the top of the UMAP/semantic-map floating window into a native <select> so users can switch albums directly from there instead of opening Settings.
• Switching goes through the existing switchAlbum() path (spinner, exit search mode, swiper rebuild, albumChanged event), so it behaves identically to the settings dropdown.
• Settings dialog album menu is untouched, as requested.

Implementation notes

• umap-floating-window.html — <span id="semanticMapTitle"> → <select id="semanticMapAlbumSelect">.
• umap.js — replaced setSemanticMapTitle() with populateSemanticMapAlbumSelect() and a one-time setupSemanticMapAlbumSelect() that wires the change listener and stops mousedown / touchstart / click / dblclick propagation on the select so the titlebar's drag and double-click-to-shade handlers don't fight the native dropdown. Removed the now-unused getCachedAlbum helper.
• umap-floating-window.css — styled #semanticMapAlbumSelect to blend into the titlebar with a custom caret and hover/focus state.

Test plan

• npm run lint clean
• npm run format:check clean
• ruff check photomap tests clean
• npm test — 320/320 passing
• pytest tests/backend — 322 passing; the only failure is the pre-existing Python 3.14 test_delete_image issue, unrelated to this branch
• Manual verification by author: switching albums from the dropdown works as advertised

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