perf(read-image-resizer): decode only first 32KB of base64 for dimension parsing

Previously decoded entire image buffer to read headers. Now slices base64
to 32KB prefix before decoding — sufficient for PNG/GIF/WebP/JPEG headers.
Dramatically reduces memory allocation for large images.

src/hooks/read-image-resizer/image-dimensions.ts

@@ -2,6 +2,9 @@ import type { ImageDimensions } from "./types"
 
 import { extractBase64Data } from "../../tools/look-at/mime-type-inference"
 
+const HEADER_BYTES = 32_768
+const HEADER_BASE64_CHARS = Math.ceil(HEADER_BYTES / 3) * 4
+
 function toImageDimensions(width: number, height: number): ImageDimensions | null {
   if (!Number.isFinite(width) || !Number.isFinite(height)) {
     return null
@@ -157,7 +160,8 @@ export function parseImageDimensions(base64DataUrl: string, mimeType: string): I
       return null
     }
 
-    const buffer = Buffer.from(rawBase64, "base64")
+    const headerBase64 = rawBase64.length > HEADER_BASE64_CHARS ? rawBase64.slice(0, HEADER_BASE64_CHARS) : rawBase64
+    const buffer = Buffer.from(headerBase64, "base64")
     if (buffer.length === 0) {
       return null
     }