Fix Morse mode HF reception, stop button, and UX guidance

Enable direct sampling (-D 2) for RTL-SDR at HF frequencies below 24 MHz
so rtl_fm can actually receive CW signals. Add startup health check to
detect immediate rtl_fm failures. Push stopped status event from decoder
thread on EOF so the frontend auto-resets. Add frequency placeholder and
help text. Fix stop button silently swallowing errors.

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

static/js/modes/morse.js

@@ -107,7 +107,14 @@ var MorseMode = (function () {
                 disconnectSSE();
                 stopScope();
             })
-            .catch(function () {});
+            .catch(function (err) {
+                console.error('Morse stop request failed:', err);
+                // Reset UI regardless so the user isn't stuck
+                state.running = false;
+                updateUI(false);
+                disconnectSSE();
+                stopScope();
+            });
     }
 
     // ---- SSE ----