Feat: Add logo, favicon and bad.webp; move images to /assets

- New logo (svg + jpg) and multi-size favicon.ico
- /favicon.ico still served at root so direct image views get the icon
- Generated missing bad.webp placeholder
- Moved limit.webp into /assets and routed error images via helper

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

assets/logo.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" fill="none" stroke-linecap="round" stroke-linejoin="round">
+  <style>
+    .f{stroke:#1a1a1a}.a{stroke:#0070f3}
+    @media (prefers-color-scheme:dark){.f{stroke:#f5f5f5}}
+  </style>
+  <!-- frame -->
+  <rect class="f" x="7" y="7" width="50" height="50" rx="13" stroke-width="4.5"/>
+  <!-- the URL "/" rising into a peak -->
+  <path class="f" d="M19 57 29 32l10 14 5-7 13 5" stroke-width="4.5"/>
+  <!-- a link as the sun -->
+  <path class="a" d="M-2-4h-3a4 4 0 0 0 0 8h3M2-4h3a4 4 0 0 1 0 8H2M-4 0h8" stroke-width="2.5" transform="translate(42 22) rotate(-45)"/>
+</svg>