fix(http): decode header values as ISO-8859-1 (#3586)

* fix(http): decode response header values as ISO-8859-1

Response header values were decoded with String::from_utf8 before being
sent over IPC. Raw UTF-8 bytes produced JS strings with code points
above 255, which new Headers() rejects as invalid ByteStrings
("Type error" on WebKit), and bytes that were not valid UTF-8 failed
the fetch outright. Decode as ISO-8859-1 instead so every byte maps to a
char 0-255, matching what browsers expose for the same response.

Fixes #2898

* fix(http): add tests for ISO-8859-1 response header decoding

Cover the #2898 Content-Disposition case and non-UTF-8 header bytes on
the same conversion fetch_send uses, so a revert to UTF-8 fails the suite.

* request headers, e2e test

---------

Co-authored-by: Lucas Nogueira <lucas@tauri.app>

examples/api/src-tauri/capabilities/base.json

@@ -11,6 +11,9 @@
         "https://tauri.app",
         {
           "url": "http://localhost:3003"
+        },
+        {
+          "url": "http://127.0.0.1:3004"
         }
       ]
     },