UI: reorder game modes (Assist first), rename Sight Reading -> Learn Notes

Internal key 'sight_reading' kept for localStorage compatibility;
TODO added at gameMode type def for future rename + migration.

src/App.tsx

@@ -277,11 +277,19 @@ function App() {
 
           <div className="header-controls">
             <div className="game-mode-toggle">
+              <button
+                className={`toggle-option ${settings.gameMode === 'assist' ? 'active' : ''}`}
+                onClick={() => setSettings(s => ({ ...s, gameMode: 'assist' }))}
+                title="Practice companion: shows played notes while you work from paper"
+              >
+                Assist
+              </button>
               <button
                 className={`toggle-option ${settings.gameMode === 'sight_reading' ? 'active' : ''}`}
                 onClick={() => setSettings(s => ({ ...s, gameMode: 'sight_reading' }))}
+                title="Learn to read notes from the staff"
               >
-                Sight Reading
+                Learn Notes
               </button>
               <button
                 className={`toggle-option ${settings.gameMode === 'ear_training' ? 'active' : ''}`}
@@ -296,13 +304,6 @@ function App() {
               >
                 Phrases
               </button>
-              <button
-                className={`toggle-option ${settings.gameMode === 'assist' ? 'active' : ''}`}
-                onClick={() => setSettings(s => ({ ...s, gameMode: 'assist' }))}
-                title="Practice companion: shows played notes while you work from paper"
-              >
-                Assist
-              </button>
             </div>
 
             <button