enhance(embed): add ability to pin color mode

[kakkokari-gtyih]

What

• You can now enforce the color mode to dark or light at the embed code author choices (of course you can choose system too).
  • Added an interface to the landing page that allows users to select a color mode.
  • data-bluesky-color-mode attribute is added for embed code (optional, default is system)

Why

Fix #7063

Additional Info

[gaearon]

I think it would be better to remove the "want to customize" panel and just do a select box ("Theme: System/Dark/Light").

[kakkokari-gtyih]

[kakkokari-gtyih]

Swapped auto with system

[kankenny]

Appreciate this feat.

[kakkokari-gtyih]

I think it would be better to remove the "want to customize" panel and just do a select box ("Theme: System/Dark/Light").

done! (See the image above for reference)

[mozzius]

Thanks - I've added some stuff on top of this in #7186, so I'll close this PR. thank you, great work!