fix(web): favicon matches nav-bar logo (monochrome outline, prefers-color-scheme)

The indigo filled square didn't match the in-page Logo component. The nav-bar
logo is a monochrome outlined rounded square + M path, currentColor on the
dark page background.

Favicon now follows the same design: outlined rect + M, stroke adapts:
  - light browser tabs  → #0A0A0B (near-black)
  - dark browser tabs   → #FAFAFA

Apple-icon stays as the indigo filled tile — iOS home-screen icons need solid
backgrounds, monochrome outlines disappear there.

apps/web/app/icon.svg

@@ -1,4 +1,10 @@
 <svg width="32" height="32" viewBox="0 0 32 32" xmlns="http://www.w3.org/2000/svg">
-  <rect width="32" height="32" rx="7" fill="#6366F1"/>
+  <style>
-  <path d="M8.5 22.5V9.5L16 16L23.5 9.5V22.5" stroke="#FFFFFF" stroke-width="2.4" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+    .stroke { stroke: #0A0A0B; }
+    @media (prefers-color-scheme: dark) {
+      .stroke { stroke: #FAFAFA; }
+    }
+  </style>
+  <rect class="stroke" x="2.5" y="2.5" width="27" height="27" rx="6" stroke-width="2.4" fill="none"/>
+  <path class="stroke" d="M8.5 22.5V9.5L16 16L23.5 9.5V22.5" stroke-width="2.4" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
 </svg>