feat(theme-selector): black/white anchors, locked ground/fg tiles, save/export/import

I made the ground default pure black and the default text pure white, the two anchors the whole theme is judged against. The palette tiles holding the current background and foreground are now locked, showing a small lock instead of the remove ×, so the contrast reference can't be deleted out from under everything.

The save/load row is relabeled and the buttons stay right-aligned: export is always a fresh download, import loads a file, and show toggles the JSON box. A save button appears once a theme name is entered and uses the File System Access API to write the same file in place on repeat saves, falling back to a download where that API isn't available.

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