fix(filters): support Buffer input in base64_encode to prevent binary data corruption (#881)

* fix: support Buffer input in base64_encode filter

When binary data (e.g. images, PDFs) is passed through the template
context as a Node.js Buffer, the base64_encode filter would call
stringify() on it first, which internally does String(value). This
triggers Buffer.toString() with the default 'utf-8' encoding, which
is a lossy conversion for non-UTF-8 byte sequences — invalid bytes
get replaced with U+FFFD, permanently destroying the original data.

The fix checks for Buffer.isBuffer() before stringify, and calls
buffer.toString('base64') directly, bypassing the lossy UTF-8
intermediate step. String inputs continue through the existing path
unchanged.

Made-with: Cursor

* fix: handle Buffer in filter layer to fix browser build

Move Buffer handling from base64-impl.ts (which gets swapped for the
browser impl at build time) into base64.ts (the filter layer). This
avoids a type error during the browser rollup build where the browser
impl only accepts string.

Also guard Buffer.isBuffer() with typeof Buffer !== 'undefined' for
safety in browser environments.

Made-with: Cursor

src/filters/base64.ts

@@ -8,7 +8,11 @@ import { FilterImpl } from '../template'
 import { stringify } from '../util'
 import { base64Encode, base64Decode } from './base64-impl'
 
-export function base64_encode (this: FilterImpl, value: string): string {
+export function base64_encode (this: FilterImpl, value: string | Buffer): string {
+  if (typeof Buffer !== 'undefined' && Buffer.isBuffer(value)) {
+    this.context.memoryLimit.use(value.byteLength)
+    return value.toString('base64')
+  }
   const str = stringify(value)
   this.context.memoryLimit.use(str.length)
   return base64Encode(str)