fix(branding): align social preview underline + URL to headline width

User feedback on banner 18b1085:
  1. Underline accent should span the WHOLE headline (was 100px under
     just "Wh" letters)
  2. GitHub URL credit should right-align to the headline's right
     edge (was anchored at the banner-edge, x=1210)

Measured the headline's actual rendered width via headless
Chromium getBBox():

    Whitelist Manager (76pt bold, letter-spacing -1.5)
    → x=520, end_x=1156.97, width=636.97

Adjustments:

- Underline rect: width 100 → 637 (spans 520→1157)
- GitHub URL text-anchor end-x: 1210 → 1157 (aligns with headline)

Both elements now share the same right-edge column, giving the
right half of the banner a clean vertical alignment from headline
to URL credit.

Re-rendered docs/icons/exports/social-preview-1280x640.png; SVG
source comments updated to record the measured end-x so future
edits don't drift from the alignment.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: RelativisticJet