Fix image upload on Linux/Wayland (WebKitGTK)

WebKitGTK can't serialize FormData with Blob objects through Tauri's
HTTP plugin, throwing "InvalidStateError: Unable to read form data
blob". Build the multipart/form-data body manually as raw bytes
instead. Also add void.cat and nostrimg.com to the HTTP scope.

src-tauri/capabilities/default.json

@@ -12,6 +12,8 @@
       "identifier": "http:default",
       "allow": [
         { "url": "https://nostr.build/**" },
+        { "url": "https://void.cat/**" },
+        { "url": "https://nostrimg.com/**" },
         { "url": "https://relay.vertexlab.io/**" }
       ]
     },