docs(design): remove SVG, use PNG logo only

SVG icon is no longer in use. public/logo.png (1024x1024 RGBA) is
the single logo asset. DESIGN.md updated accordingly: SVG snippet
removed, wordmark pattern and <head> references updated to PNG.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/logo.svg

@@ -1,12 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" fill="none">
-  <circle cx="16" cy="16" r="16" fill="#0f172a"/>
-  <circle cx="16" cy="16" r="5" fill="#38bdf8"/>
-  <circle cx="8"  cy="8"  r="3" fill="#38bdf8" opacity=".7"/>
-  <circle cx="24" cy="8"  r="3" fill="#38bdf8" opacity=".7"/>
-  <circle cx="8"  cy="24" r="3" fill="#38bdf8" opacity=".7"/>
-  <circle cx="24" cy="24" r="3" fill="#38bdf8" opacity=".7"/>
-  <line x1="11" y1="11" x2="16" y2="16" stroke="#38bdf8" stroke-width="1.5" opacity=".5"/>
-  <line x1="21" y1="11" x2="16" y2="16" stroke="#38bdf8" stroke-width="1.5" opacity=".5"/>
-  <line x1="11" y1="21" x2="16" y2="16" stroke="#38bdf8" stroke-width="1.5" opacity=".5"/>
-  <line x1="21" y1="21" x2="16" y2="16" stroke="#38bdf8" stroke-width="1.5" opacity=".5"/>
-</svg>