feat(branding): GitHub social preview banner (1280×640)

Adds the SVG source + rendered PNG for the GitHub social preview
image — the OG card that displays when someone shares the repo
URL in Slack, Discord, Twitter, or LinkedIn.

Composition matches the launcher-tile design language so the brand
reads consistently across surfaces:

- Slate-grey #2E3A47 backdrop (same as the launcher tile)
- Aqua shield + document + check badge composition (same geometry
  as docs/icons/appIcon-light.svg, scaled to 400×400 on the left)
- "Whitelist Manager" headline (76pt bold, white) with an aqua
  underline accent picking up the shield color
- "for Splunk Enterprise Security" subtitle (38pt regular)
- "Audit-traced detection-rule whitelisting" value-prop line
  (30pt regular, muted slate)
- Repo URL credit in the bottom-right corner

Files:

- docs/icons/social-preview.svg (114 LOC) — editable SVG master,
  kept alongside appIcon-light.svg + appIcon-dark.svg so future
  re-renders / re-brands can be done from a single canonical source
- docs/icons/exports/social-preview-1280x640.png (40 KB) — the
  uploadable artifact for GitHub's repo Settings → "Social preview"

Re-render command (uses scripts/svg2png.js, no extra install):

    node scripts/svg2png.js docs/icons/social-preview.svg \
      docs/icons/exports/social-preview-1280x640.png 1280 640

To deploy:

1. Go to https://github.com/RelativisticJet/wl_manager/settings
2. Scroll to "Social preview"
3. Upload docs/icons/exports/social-preview-1280x640.png

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

Author: RelativisticJet