fix(fonts): skip preview API call for BDF bitmap fonts

The font preview endpoint explicitly rejects .bdf files (glyph rendering
not implemented server-side), returning 400. The JS didn't know this and
called the endpoint for every selected font, causing a noisy 400 on load.

Guard added in updateFontPreview(): if the selected font ends in .bdf,
show "Preview not available for BDF bitmap fonts" and return early instead
of hitting the API.

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

web_interface/templates/v3/partials/fonts.html

@@ -843,6 +843,14 @@ async function updateFontPreview() {
         return;
     }
 
+    // BDF bitmap fonts cannot be rendered server-side — skip the API call
+    if (family.toLowerCase().endsWith('.bdf')) {
+        previewImage.style.display = 'none';
+        loadingText.style.display = 'block';
+        loadingText.textContent = 'Preview not available for BDF bitmap fonts';
+        return;
+    }
+
     // Show loading state
     loadingText.textContent = 'Loading preview...';
     loadingText.style.display = 'block';