[Fix] Use UTF-8 buffer for HTML URL rewriting

[Fix] Use UTF-8 buffer for HTML URL rewriting

The HTML parser calculates attribute value offsets from the UTF-8
buffer (utf_raw_content), but URL rewriting was incorrectly applying
patches to the MIME-decoded buffer (parsed). When charset conversion
occurs (e.g., from ISO-8859-1 to UTF-8), the same character can have
different byte lengths, causing incorrect patch positions.

This commit ensures all URL rewriting operations use the UTF-8 buffer
consistently, preventing corruption with non-ASCII characters.