[fix](trx-backend-ft817): mark AMC unsupported in FT-817 encoder

Add RigMode::AMC to the None return arm of encode_mode so that
attempting to set C-QUAM on the FT-817 returns a descriptive error
rather than a compile-time non-exhaustive match.

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-ft817/src/lib.rs

@@ -603,7 +603,7 @@ fn encode_mode(mode: &RigMode) -> Option<u8> {
         RigMode::FM => 0x08,
         RigMode::DIG => 0x0A,
         RigMode::PKT => 0x0C,
-        RigMode::AIS | RigMode::VDES | RigMode::Other(_) => return None,
+        RigMode::AIS | RigMode::VDES | RigMode::AMC | RigMode::Other(_) => return None,
     })
 }