[fix](trx-server): auto-generate rig IDs when not specified in config

When rigs are configured without explicit IDs in TOML, they now
receive auto-generated IDs based on model name and index (e.g.,
"ft817_0", "soapysdr_1"). Previously, the default empty ID prevented
auto-generation from triggering.

Changes:
- Set RigInstanceConfig default id to empty string (was "default")
- This allows resolved_rigs() auto-generation to trigger for all rigs
- Legacy single-rig path still gets explicit "default" ID
- Auto-generated IDs now appear in HTTP API /rigs endpoint
- Rig picker now displays auto-IDs correctly

The fix ensures that rigs without explicit ids get proper identifiers
that appear in the protocol and frontend selectors.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-server/src/config.rs

@@ -54,7 +54,7 @@ pub struct RigInstanceConfig {
 impl Default for RigInstanceConfig {
     fn default() -> Self {
         Self {
-            id: "default".to_string(),
+            id: String::new(),  // Empty by default so auto-generation triggers in resolved_rigs()
             name: None,
             rig: RigConfig::default(),
             behavior: BehaviorConfig::default(),

src/trx-server/src/listener.rs

@@ -461,6 +461,7 @@ mod tests {
         let _state_tx = state_tx;
         let handle = RigHandle {
             rig_id: "default".to_string(),
+            display_name: "Default Rig".to_string(),
             rig_tx,
             state_rx,
             audio_port: 4531,