[feat](trx-wefax): auto-detect active signal and show live decode

Add signal-level detection that monitors luminance variance to auto-start
receiving when tuning in mid-image (~3s of sustained modulated signal),
matching fldigi's "strong image signal" detection. Reduce APT sustain
to 1.0s (2 windows) matching fldigi. Emit initial "Idle — scanning"
state event so the frontend shows the decoder is processing audio.
Add tracing instrumentation for luminance stats and tone analysis.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/wefax.js

@@ -253,7 +253,8 @@ window.onServerWefaxProgress = function (msg) {
   if (msg.state && !msg.line_data) {
     if (wefaxDom.status) {
       wefaxDom.status.textContent = msg.state;
-      wefaxDom.status.style.color = 'var(--text-accent)';
+      // Highlight active states, dim idle/scanning.
+      wefaxDom.status.style.color = msg.state.indexOf('Idle') === 0 ? '' : 'var(--text-accent)';
     }
     return;
   }