Style all back/switch/stop buttons as proper pill buttons

Replaces the plain muted text style with a white card button: border, shadow, hover lift, and transition — consistent across Switch, Back, Stop, and Cancel labels throughout the student app.

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

public/styles.css

@@ -178,9 +178,23 @@ body:has(#view-home:not([hidden])) .page-confetti { display: block; }
 
 /* ---------- kid home ---------- */
 .back-btn {
-  display: inline-block;
-  background: none; border: none; color: var(--muted);
-  font-size: 1rem; cursor: pointer; margin-bottom: 1rem;
+  display: inline-flex; align-items: center; gap: 0.3rem;
+  background: var(--card);
+  border: 1.5px solid #d8d2c0;
+  border-radius: 10px;
+  color: var(--muted);
+  font-size: 0.9rem; font-weight: 600; font-family: inherit;
+  padding: 0.4rem 0.9rem;
+  cursor: pointer; margin-bottom: 1rem;
+  box-shadow: 0 2px 6px rgba(45,49,66,0.07);
+  transition: background 0.15s, color 0.15s, transform 0.12s, box-shadow 0.15s;
+}
+.back-btn:hover {
+  background: #fff;
+  color: var(--ink);
+  border-color: #bdb8aa;
+  box-shadow: 0 4px 12px rgba(45,49,66,0.12);
+  transform: translateY(-1px);
 }
 #kid-greeting { font-size: 2rem; margin: 1rem 0; clear: both; }