Assist: drop flickering hint paragraph; center tuner in its frame

The 'Play a note' paragraph toggled with every detection gap and shifted
the layout — removed (incl. unused .assist-hint rule). TuningMeter is a
fixed-width 260px block; the tuner tool's body row now centers it.

src/App.css

@@ -1190,13 +1190,6 @@ button {
   overflow-x: auto;
 }
 
-.assist-hint {
-  font-size: 12px;
-  color: var(--color-text-muted);
-  margin: 0;
-  text-align: center;
-}
-
 /* ---- Transport step group (preview stepping) ---------------------------- */
 .transport-step-group {
   display: flex;

src/components/AssistMode.tsx

@@ -97,13 +97,6 @@ export const AssistMode: React.FC<AssistModeProps> = ({ pitchData, windowWidth }
                     />
                 </div>
             )}
-
-            {!pitchData && (
-                <p className="assist-hint">
-                    Play a note — it shows here. Mic is{' '}
-                    {settings.showTuningMeter ? 'on' : 'controlled by the mic button below'}.
-                </p>
-            )}
         </div>
     );
 };

src/components/Controls.tsx

@@ -86,7 +86,7 @@ export const Controls: React.FC<ControlsProps> = ({ currentPitch }) => {
                         </button>
                     </div>
                     {settings.showTuningMeter && (
-                        <div style={{ paddingTop: '8px', borderTop: '1px solid color-mix(in srgb, var(--color-text-main) 10%, transparent)' }}>
+                        <div style={{ paddingTop: '8px', borderTop: '1px solid color-mix(in srgb, var(--color-text-main) 10%, transparent)', display: 'flex', justifyContent: 'center' }}>
                             <TuningMeter
                                 cents={currentPitch ? currentPitch.cents : null}
                                 noteName={currentPitch ? currentPitch.note : null}