fix(brand): yellow V in TopNav wordmark

Wordmark was plain text 'WeVisto'; the V should be yellow #f0d000
(the brand accent) — consistent with the logo.svg and every other
'WeVisto' rendering in the design.

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

frontend/src/components/TopNav.vue

@@ -3,7 +3,7 @@
     <RouterLink to="/" class="top-nav__brand">
       <span class="top-nav__mark"><img :src="markUrl" alt=""></span>
       <span class="top-nav__title-group">
-        <span class="top-nav__wordmark">WeVisto</span>
+        <span class="top-nav__wordmark">We<span class="top-nav__wordmark-v">V</span>isto</span>
         <span class="top-nav__sub">— {{ activeSub }}</span>
       </span>
     </RouterLink>
@@ -122,6 +122,8 @@ const tabs: Tab[] = [
     font-weight: 800;
     letter-spacing: -0.01em;
     color: var(--color-text);
+
+    &-v { color: #f0d000; }  // brand yellow — survives every theme
   }
 
   // Italic subtitle "— home / library / settings" beside the wordmark.