fix(exampleSite): use Latin T glyph for avatar (font-portable)

Headless renderers and many systems lack Japanese fonts to render the
kanji glyph used previously. T is universally available and reads as
a Tien-Nguyen demo monogram.

themes/bonsai/exampleSite/static/images/avatar.svg

@@ -1,4 +1,4 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" width="200" height="200">
   <circle cx="100" cy="100" r="100" fill="#8b3a2b"/>
-  <text x="100" y="140" font-family="Georgia, 'Hiragino Mincho ProN', 'Yu Mincho', serif" font-size="120" font-weight="600" fill="#f4efe6" text-anchor="middle">盆</text>
+  <text x="100" y="138" font-family="Georgia, serif" font-size="110" font-weight="600" fill="#f4efe6" text-anchor="middle">T</text>
 </svg>