feat(theme-selector): full category coverage, elements rename, visible tile borders

I renamed the assignments column "category" to "elements" and gave every language sample a token for each category that applies to it, so clicking an element name flashes real code in any language: Python gained a regexp, Elisp/Go/TypeScript/C each a builtin, and Go and Shell an escape. The categories a language genuinely lacks (Elisp has no preprocessor, Shell no types) fall back to flashing the example cell.

The save area header is now "export, import, and save", and the palette tiles and the picker swatch carry a visible border so a tile that matches the page background still reads.

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