[fix](trx-backend-ft450d): mark AMC unsupported in FT-450D encoder

Add a RigMode::AMC arm to encode_mode returning an Err so that
attempting to set C-QUAM on the FT-450D returns a descriptive error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/trx-backend/trx-backend-ft450d/src/lib.rs

@@ -519,6 +519,7 @@ fn encode_mode(mode: &RigMode) -> DynResult<char> {
         RigMode::CWR => Ok('7'),
         RigMode::PKT => Ok('9'),
         RigMode::WFM => Ok('4'),
+        RigMode::AMC => Err("Unsupported mode for FT-450D".into()),
         RigMode::Other(_) => Err("Unsupported mode for FT-450D".into()),
     }
 }