Add the Endless theme family

[brsbl]

What changed

This adds the complete Endless theme plugin with two related themes: a restrained monochrome treatment and an artwork-derived color treatment. The palette values are sampled from the album artwork and then solved against explicit contrast floors, so the numbers are derived and traceable rather than hand-picked.

The plugin builds against the released @get-bb/plugin-sdk 0.4.8 toolchain while retaining the honest compatibility floor engines.bbPluginSdk: ^0.4.1 and engines.bb: >=0.0.34.

Generator included

The three generator sources now live under plugins/endless/build/ with repo-relative paths. Regenerating from that moved source reproduces both committed theme files byte-for-byte:

• endless.css: a05f79cc8ddc883a1ba7542fa98e79aff7c4fe8d914385440c504b786794906b
• endless-color.css: a77e182f318779feb8b0f65541ca65421004a354f00890da90dc501afb82615f

This PR is a fresh commit on current origin/main and supersedes the unpushed bb/endless-theme-plugin branch. That older branch was not rebased because its history also carried unrelated Timeline Comments commits.

Verification

• npm run check --workspace=bb-plugin-endless
• node tooling/check-repository.mjs
• Byte-for-byte generator reproduction for both CSS outputs
• Mobile, narrow, normal, and ultrawide fixture captures; light and dark modes inspected for both theme variants

Evidence

Before — merge base with the default theme

After — this branch with Endless Color

Full visual evidence

BB-Thread-ID: thr_tkwwfu5nas