feat(viewer): use the eegdash long wordmark + larger logo in embed

Swap embed-mode brand from the sphere-only glyph to the full
``eegdash_long.svg`` wordmark so the iframe carries a readable brand
lockup on the host docs page. Scales the logo to ~26px tall (was 28px
square) and bumps the header to 44px so the wider mark has breathing
room on both sides.

Cache-bust: styles.css?v=6.

Author: bruAristimunha