Fix WeFax start/stop/SSE reliability

- Replace blocking stdout.read() with select()-based non-blocking reads
  so the decode thread responds to stop within 0.5s
- Make stop() non-blocking by releasing the lock before terminating the
  process and removing the redundant wait()
- Move initial scanning SSE event from start() into the decode thread so
  it fires after the frontend EventSource connects
- Update frontend stop() to give immediate UI feedback before the fetch

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

static/js/modes/wefax.js

@@ -205,16 +205,20 @@ var WeFax = (function () {
     }
 
     function stop() {
+        // Immediate UI feedback before waiting for backend response
+        state.running = false;
+        updateButtons(false);
+        setStatus('Stopping...');
+        disconnectSSE();
+
         fetch('/wefax/stop', { method: 'POST' })
             .then(function (r) { return r.json(); })
             .then(function () {
-                state.running = false;
-                updateButtons(false);
                 setStatus('Stopped');
-                disconnectSSE();
                 loadImages();
             })
             .catch(function (err) {
+                setStatus('Stopped');
                 console.error('WeFax stop error:', err);
             });
     }