fix: refinements and bugfixes for new themes

[marcalexiei]

Found while reviewing #457.

1. Dark themes rendered a near-white border.
   dark_github / dark_github_repocard had no border_color and fell back to the default theme's e4e2e2.
   Set GitHub's borders on all four new themes: d1d9e0 light, 3d444d dark.

2. Excluding default / default_repocard broke the theme picker.
   Home.tsx initializes to one of them and treats it as "emit no theme= param", so stage 3 opened with nothing selected and the theme-less URL became unreachable.

3. Paired themes were shown for the wrong card type.
   The picker now takes isRepoCard and lists only the matching variant, hiding duplicates while keeping each card type's default visible.

4. Theme README generator only knew the default pair, so the new themes would land in the wrong tables.

5. 3 and 4 share a rule, kept duplicated on purpose: which themes to advertise per card type is presentation, not rendering, so it stays out of core's API.
   Both copies carry Keep in sync comments pointing at each other.

6. Dead docs anchor left by the Responsive Card Theme ➡️ Light and Dark Mode rename.

7. Core gains one export: the ThemeName type, replacing the private duplicate ThemeNames in cards/types.ts and the keyof typeof themes casts in color.ts and the picker.

[vercel]

@marcalexiei is attempting to deploy a commit to the martin-mfg's projects Team on Vercel.

A member of the Team first needs to authorize it.

[martin-mfg]

Wow, I missed a lot of things. 🙈