feat: canonical theme picker and repo remotes

[LizAinslie]

Summary

• Canonical theme picker: ThemePicker.astro adopts the Keel switcher look — a compact popup with <ul class="menu menu-sm">, one menu-title per site-theme family (via groupChromeThemes), swatch dots, and short Catppuccin labels. Code themes are now the curated list from CODE_THEME_OPTIONS / codeThemeOptions(config) instead of every site theme. The chrome preference contract (data-pref, data-label, kdc:*, applyPrefs/syncControls, data-theme/data-code-theme, follow) is unchanged.
• Source control: RepoConfig gains remotes?: FooterLink[]. defineDocsChrome derives scm from repo.remotes (deduplicated by href, order preserved) or a single { label: 'Source', href: repo.url }; explicit scm still wins. ScmMenu renders one link as a single icon link and several as the existing popover menu.
• Tests for both derivations and the curated code-theme list; README + CHANGELOG updated.

Changelog

Added

• Curated code themes. Core exports CodeThemeOption, CODE_THEME_OPTIONS (follow plus the Catppuccin flavours, Nord and the Kolektiv palettes) and codeThemeOptions(config), which restricts the list to the themes a site offers (so themeFamilies applies) and appends any config.themes. ThemePicker renders site themes grouped by family — one menu-title per family, Catppuccin shown by flavour (for example Mocha) — with the curated code themes below follow.
• RepoConfig.remotes (FooterLink[]). When supplied, defineDocsChrome derives scm from it, deduplicated by href and order-preserving; otherwise it derives a single { label: 'Source', href: repo.url }. An explicit scm still wins. deriveScm(repo) is exported.

Changed

• ScmMenu renders a single icon link when the deduplicated scm list has one entry (previously it always opened a popover); several links keep the popover menu and zero links render nothing. defineDocsChrome no longer emits the duplicate Source / yuri.capital pair for repo.url.

Verification

• pnpm test, pnpm typecheck, pnpm build, playground build all pass.
• Playground before/after diff limited to the theme picker, the SCM control, and the Tailwind utility classes those markup changes pull in.