Fix preview file size calculation and format conversion

frontend/src/lib/preview.ts

@@ -38,10 +38,29 @@ export async function generateClientPreview(
           ctx.drawImage(img, 0, 0, width, height);
         }
 
-        // Convert to data URL with quality
+        // Convert to data URL with quality - fix MIME type mapping
         const quality = options.quality / 100;
-        const mimeType = `image/${options.format === 'jpeg' ? 'jpeg' : 'png'}`;
-        const dataUrl = canvas.toDataURL(mimeType, quality);
+        let mimeType: string;
+        
+        // Map format to proper MIME type
+        switch (options.format) {
+          case 'jpeg':
+            mimeType = 'image/jpeg';
+            break;
+          case 'webp':
+            mimeType = 'image/webp';
+            break;
+          case 'png':
+          default:
+            mimeType = 'image/png';
+            break;
+        }
+        
+        // For PNG, quality doesn't apply in Canvas API (always lossless)
+        // For JPEG and WebP, quality matters
+        const dataUrl = options.format === 'png' 
+          ? canvas.toDataURL(mimeType)
+          : canvas.toDataURL(mimeType, quality);
         
         resolve(dataUrl);
       } catch (error) {
@@ -183,12 +202,26 @@ function getPositionOffset(
 
 /**
  * Estimate file size from data URL
+ * More accurate calculation that accounts for base64 overhead
  */
 export function estimateSize(dataUrl: string): number {
-  const base64 = dataUrl.split(',')[1];
+  const parts = dataUrl.split(',');
+  if (parts.length < 2) return 0;
+  
+  const base64 = parts[1];
   if (!base64) return 0;
-  // Base64 is ~33% larger than binary, so divide by 1.33
-  return Math.ceil((base64.length * 3) / 4);
+  
+  // Remove padding characters for accurate calculation
+  const withoutPadding = base64.replace(/=/g, '');
+  
+  // Base64 encoding: 3 bytes -> 4 characters
+  // So to get original bytes: (length * 3) / 4
+  const bytes = (withoutPadding.length * 3) / 4;
+  
+  // Account for padding bytes if present
+  const paddingCount = base64.length - withoutPadding.length;
+  
+  return Math.round(bytes - paddingCount);
 }
 
 /**