fix(gui): transcode non-UTF-8 text uploads to UTF-8

Strict UTF-8 probing rejected GBK/Big5/Shift-JIS/UTF-16 text files as
binary and let legacy-encoded text reach the model as mojibake. Classify
uploads into utf8/needs-transcode/binary (tolerating multibyte chars cut
at the 32KiB probe boundary), transcode staged copies and native
attachment inlining to UTF-8 via encoding_rs + chardetng, and leave
in-workspace source files untouched.

Author: su-fen

Cargo.lock

@@ -62,6 +62,8 @@ portable-pty = "0.9.0"
 wait-timeout = "0.2.1"
 russh = "0.62.2"
 russh-sftp = "2.3.0"
+encoding_rs = "0.8.35"
+chardetng = "0.1.17"
 
 [target.'cfg(target_os = "macos")'.dependencies]
 objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "NSButton", "NSControl", "NSView", "NSWindow"] }