fix: move logo_vertical svgs to extension root so splash watermark loads

SidePanel.tsx default-state welcome screen uses <img src="/logo_vertical.svg">
as an opacity-0.25 watermark. The file was in pages/side-panel/public/, so
Vite emitted it to dist/side-panel/logo_vertical.svg while the absolute
path requested chrome-extension://<id>/logo_vertical.svg — 404. Same class
of bug that #41 fixed for kk.webp and brand icons.

Moving both vertical logos to chrome-extension/public/ lands them at the
dist root and matches the working /kk-logo.png / /kk.webp convention.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: BitHighlander