fix(icons): slate-grey #2E3A47 backdrop — defeat Splunk launcher green leak

The launcher tile rendered with a green halo around the aqua shield
because Splunk's launcher CSS applies `background-color: #65A637`
(their brand green) directly to every app-icon <img> element. Our
original PNG had transparent corners outside the shield silhouette
(omitBackground: true at export time), so Splunk's green leaked
through.

Iterated through three backdrop candidates before landing here:
  1. White corners — visually clean but stood out as the only white
     tile in the launcher row (Search & SSG are green, Upgrade
     Readiness is grey; ours looked unthemed)
  2. Shield-fills-frame — extended shield path to viewBox edges,
     but the curved shield bottom still left transparent triangles
     in the bottom-left/bottom-right corners (smaller green leak,
     not zero)
  3. Slate grey #2E3A47 — opaque rect filling the full 256×256
     viewBox under the original inset shield; strong aqua-on-dark
     contrast, theme-stable, distinct from neighboring tiles
     without competing with Splunk's brand color

Changes:

- docs/icons/appIcon-light.svg + appIcon-dark.svg: prepend
  `<rect width="256" height="256" fill="#2E3A47"/>` as the first
  child of the <svg>, BEFORE the shield path. Light + dark variants
  share the same backdrop (slate works in both themes).
- Re-render the 8 launcher PNGs (4 in static/, 4 in appserver/static/)
  + the 2 Splunkbase exports (144, 512) from the updated SVGs.

Verified on wl_manager_test container:

- MD5 of served PNG matches host source byte-for-byte
- Canvas pixel-probe: all 4 corners now `rgba(46,58,71,255)` (slate
  grey opaque) — was `rgba(0,0,0,0)` transparent → Splunk green
  leaked through
- Launcher screenshot shows our slate tile sitting cleanly among the
  Search/SSG green tiles and Upgrade Readiness grey tile — visually
  distinctive premium aesthetic, no leaks

Pairs with commit e536d01 (the package.sh wildcard fix) — together
these two commits ensure the launcher tile actually looks correct
in customer .spl installs.

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

Author: RelativisticJet