fix: accept firmware uploads independent of part MIME type

Chrome on macOS labels .bin files application/macbinary. Searching for application/octet-stream mistakes the image bytes for more part headers and aborts when the preamble buffer fills.

Find the multipart header terminator instead, after the HTTP separator retained by handle_post. Keep authentication, CRC checks and staging limits unchanged. Cover MacBinary, an omitted part Content-Type, and a split header terminator with exact staged-byte checks.

CHANGELOG.md

@@ -25,6 +25,10 @@
 
 ## Fixed
 
+- Web UI
+  - Accept firmware uploads regardless of the file part's MIME type, including
+    `application/macbinary` sent by Chrome on macOS for `.bin` files.
+
 - Config
   - A startup configuration line longer than the command buffer no longer stops the replay:
     the offending line is skipped and the following lines are still applied.