[fix](trx-backend-soapysdr): add AMC to supported_modes capabilities list

RigMode::AMC was implemented but omitted from the supported_modes vec,
so the HTTP frontend never received it in capabilities and the mode
selector did not show AMC-QUAM.

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-soapysdr/src/lib.rs

@@ -228,6 +228,7 @@ impl SoapySdrRig {
                     RigMode::CW,
                     RigMode::CWR,
                     RigMode::AM,
+                    RigMode::AMC,
                     RigMode::WFM,
                     RigMode::FM,
                     RigMode::AIS,