feat(branding): default both site logos to assets/img/logo.png

- Point 站点 Logo / 暗色模式 Logo schema defaults (PHP + VUE) and the
  Settings::logo_url() fallback at assets/img/logo.png.
- Point the footer night image default at the same file.
- Remove logo_dark.png and the now-unreferenced assets/icons/default-logo.svg
  along with the private default_asset_url() helper that was its only reader.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/icons/default-logo.svg

@@ -1,9 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 60" width="200" height="60">
-  <defs>
-    <linearGradient id="g" x1="0%" y1="0%" x2="100%" y2="100%">
-      <stop offset="0%" style="stop-color:#f04494"/>
-      <stop offset="100%" style="stop-color:#ae53f3"/>
-    </linearGradient>
-  </defs>
-  <text x="10" y="42" font-family="system-ui,-apple-system,sans-serif" font-size="32" font-weight="700" fill="url(#g)">Jio</text>
-</svg>