feat(viewer): embed-mode polish — neutral bg, eegdash logo, region legend

Three changes based on feedback when the iframe ships on eegdash.org
dataset pages:

- Drop the cream --bg in embed mode (transparent background), so the
  iframe blends with the host docs page instead of reading as a
  foreign panel.
- Add an eegdash logo anchor top-right, linking to eegdash.org.
  Hidden outside embed mode (the standalone viewer has its own brand).
- Add a compact region legend overlay at the stage bottom (FRONTAL /
  CENTRAL / PARIETAL / OCCIPITAL / REFERENCE / EGI / OTHER) — only
  the regions actually present in the loaded montage are shown.
  fNIRS montages get a Source/Detector key instead of regions.
  This replaces the left-rail legend which embed mode hides, so
  colors finally read as meaningful inline.

Cache-bust: styles.css?v=5.

Author: bruAristimunha