docs: fix palette explorer export and frame the palettes page

[johnleider]

Problem

The /guide/features/palettes page rated poorly for being confusing. Root cause was the interactive explorer at the top of the page:

• Copy Config emitted dead code. onExport produced a colors block where every line was commented out — pasting the output gave a theme with no colors defined (silent no-op). The click path was worse: it emitted bare token strings with no role key, which are a syntax error once uncommented.
• The widget had no instructions. It's the first thing on the page, dropped in after a single sentence, with no explanation of click-to-copy, what Copy Config does, or why the grid flips orientation on Material.
• Static-vs-generator and the {palette.…} token syntax were never framed before the widget.

Changes

• onExport now emits real, uncommented config. Clicked swatches map to theme roles (primary, secondary, …) in click order; the no-click default seeds primary/secondary from hues that actually exist in the selected palette (the old default hardcoded blue.500/slate.600, which don't exist outside Tailwind).
• In-widget hint explaining click-to-copy → Copy Config.
• Framing prose above the explorer: static vs generator forms, the token-reference syntax with a link to Theming, and a note that shade-heavy palettes (Material) render transposed.

Verification

• Export code-gen validated in isolation — both the no-click and multi-click branches produce valid, resolvable config.
• vue-tsc on apps/docs: 0 errors. ESLint clean.
• Not yet driven in a live browser — the generated-string logic is verified, but a click-through confirmation is worth doing before merge.