feat: add rtl_tcp remote SDR support to aprs, morse, and dsc routes

Closes #164. Only pager and sensor routes supported rtl_tcp connections.
Now aprs, morse, and dsc routes follow the same pattern: extract
rtl_tcp_host/port from the request, skip local device claiming for
remote connections, and use SDRFactory.create_network_device(). DSC also
refactored from manual rtl_fm command building to use SDRFactory's
builder abstraction. Frontend wired up for all three modes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/ais_dashboard.html

@@ -1179,6 +1179,19 @@
             const isAgentMode = typeof aisCurrentAgent !== 'undefined' && aisCurrentAgent !== 'local';
             dscCurrentAgent = isAgentMode ? aisCurrentAgent : null;
 
+            // Check for remote SDR (only for local mode)
+            const remoteConfig = (!isAgentMode && typeof getRemoteSDRConfig === 'function')
+                ? getRemoteSDRConfig() : null;
+            if (remoteConfig === false) return; // Validation failed
+
+            const requestBody = { device, gain };
+
+            // Add rtl_tcp params if using remote SDR
+            if (remoteConfig) {
+                requestBody.rtl_tcp_host = remoteConfig.host;
+                requestBody.rtl_tcp_port = remoteConfig.port;
+            }
+
             // Determine endpoint based on agent mode
             const endpoint = isAgentMode
                 ? `/controller/agents/${aisCurrentAgent}/dsc/start`
@@ -1187,7 +1200,7 @@
             fetch(endpoint, {
                 method: 'POST',
                 headers: { 'Content-Type': 'application/json' },
-                body: JSON.stringify({ device, gain })
+                body: JSON.stringify(requestBody)
             })
             .then(r => r.json())
             .then(data => {