:root{--bg:#f6f6f6;--panel:#fff;--card:#fafafa;--text:#171717;--muted:#6b7280;--border:#dedede;--accent:#111827;--accent-text:#fff;--accent-soft:#f0f1f3}
*{box-sizing:border-box}
body{margin:0;font-family:Inter,system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;background:var(--bg);color:var(--text)}
button,input,select,textarea{font:inherit}
button{cursor:pointer}
.prototype-root{width:min(100%,760px);margin:0 auto;padding:10px}
.phone{max-width:430px;margin:0 auto;border:1px solid var(--border);border-radius:24px;overflow:hidden;background:var(--panel);min-height:100vh}
.screen{min-height:100vh;display:flex;flex-direction:column}
.header{display:flex;align-items:center;justify-content:space-between;border-bottom:1px solid var(--border);padding:14px 16px}
.screen-body{flex:1;padding:16px}
.bottom-nav{display:grid;grid-template-columns:repeat(3,1fr);border-top:1px solid var(--border);padding:8px;gap:4px;position:sticky;bottom:0;background:var(--panel)}
.bottom-nav button{border:0;background:transparent;padding:10px 6px;border-radius:10px;color:var(--muted)}
.bottom-nav button[data-active="true"]{color:var(--accent);font-weight:700;background:var(--accent-soft)}
.title{font-size:20px;font-weight:700}
.subtitle{margin-top:4px;color:var(--muted);font-size:14px}
.muted{color:var(--muted)}
.small{font-size:14px}
.stack{display:grid;gap:14px}
.grid-2{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:12px}
.btn{width:100%;border-radius:14px;padding:13px 14px;border:1px solid var(--border);background:var(--panel);color:var(--text);text-align:left}
.btn.primary{background:var(--accent);color:var(--accent-text);border-color:var(--accent)}
.btn.center{text-align:center}
.btn.tertiary{border:0;background:transparent;color:var(--muted);text-decoration:underline;text-align:center;padding:8px 4px}
.photo-placeholder{aspect-ratio:4/3;border:1px solid var(--border);border-radius:14px;display:grid;place-items:center;background:var(--card);color:var(--muted);padding:16px;text-align:center}
.math-box{font-family:ui-monospace,SFMono-Regular,Menlo,Monaco,Consolas,monospace;font-size:16px;border:1px solid var(--border);border-radius:16px;padding:14px;background:var(--card);white-space:pre-wrap}
.student-error{border-left:3px solid var(--accent)}
.callout{background:var(--accent-soft);border-radius:16px;padding:14px}
.badge{display:inline-flex;align-items:center;width:fit-content;padding:6px 10px;border-radius:999px;background:var(--accent-soft);font-size:13px;font-weight:600}
.progress-row{display:flex;gap:12px}
.progress-dot{width:8px;height:8px;border-radius:999px;background:var(--accent);margin-top:7px;flex:0 0 auto}
input,select,textarea{width:100%;border:1px solid var(--border);border-radius:12px;padding:12px;background:var(--panel);color:var(--text)}
textarea{resize:vertical}
.hidden{display:none!important}
.history-item{width:100%;text-align:left;border:1px solid var(--border);background:var(--panel);border-radius:16px;padding:14px}
.history-top{display:flex;justify-content:space-between;gap:12px;align-items:flex-start}
.chat-row{display:flex}
.chat-row.user{justify-content:flex-end}
.chat-row.assistant{justify-content:flex-start}
.bubble{max-width:88%;border-radius:16px;padding:12px;border:1px solid var(--border);background:var(--panel)}
.chat-row.assistant .bubble{background:var(--accent-soft);border-color:transparent}
.dev-status{position:fixed;right:8px;top:8px;z-index:50;background:var(--panel);border:1px solid var(--border);border-radius:999px;padding:5px 9px;font-size:11px;color:var(--muted)}
@media(max-width:520px){.prototype-root{padding:0}.phone{border:0;border-radius:0}.grid-2{grid-template-columns:1fr}}