@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=Source+Serif+4:opsz,wght@8..60,450;8..60,500&display=swap');
:root{--bg:#f6f7f9;--surface:#fff;--surface-2:#f1f3f5;--text:#18202a;--muted:#525d6b;--line:#d8dee4;--accent:#3157d5;--accent-soft:#edf1ff;--shadow:0 8px 28px rgba(20,30,45,.06);--radius:12px}
*,*::before,*::after{box-sizing:border-box}[hidden]{display:none!important}html{scroll-behavior:smooth}body{margin:0;background:var(--bg);color:var(--text);font-family:Inter,system-ui,-apple-system,Segoe UI,sans-serif}
/* UPSC PhysicsHub shared site shell — navigation, page headers and global utility UI. */
.site-nav{background:var(--surface,#fff);border-bottom:1px solid var(--line,#d8dee4);display:flex;align-items:center;padding:0 28px;gap:4px;height:52px;position:relative;z-index:1100}
.site-nav-brand{font-size:20px;font-weight:700;color:var(--text,#18202a);text-decoration:none;letter-spacing:-.02em;margin-right:12px;white-space:nowrap}
.site-nav-brand .brand-prefix, .brand-inline .brand-prefix{color:#64748b;font-weight:500}
.site-nav-brand .brand-suffix, .brand-inline .brand-suffix{color:var(--accent,#3157d5);font-weight:700}
.brand-inline{font-weight:700;color:var(--text,#18202a);letter-spacing:-.01em}
.nav-links{display:flex;flex:1;justify-content:flex-start;align-items:center}.nav-links-inner{display:flex;flex:1;align-items:center;gap:4px}
.nav-tab{display:inline-flex;align-items:center;gap:6px;height:52px;padding:0 14px;border:none;background:transparent;color:var(--muted,#525d6b);font:inherit;font-size:13px;font-weight:600;cursor:pointer;text-decoration:none;border-bottom:2px solid transparent;transition:color .15s,border-color .15s;white-space:nowrap}
.nav-tab:hover{color:var(--text,#18202a);border-bottom-color:var(--line,#d8dee4)}
.nav-tab.active{color:var(--accent,#3157d5);border-bottom-color:var(--accent,#3157d5)}
.nav-tab.disabled,.nav-tab[aria-disabled="true"]{color:#aab0b8;cursor:not-allowed;pointer-events:none}
.nav-tab svg{width:15px;height:15px;fill:none;stroke:currentColor;stroke-width:2;stroke-linecap:round;stroke-linejoin:round}
.nav-feedback-btn{display:inline-flex;align-items:center;justify-content:center;gap:6px;height:36px;padding:0 12px;margin-left:auto;border-radius:999px;border:1px solid var(--line,#d8dee4);background:var(--surface,#fff);color:var(--muted,#525d6b);font:inherit;font-size:12.5px;font-weight:600;text-decoration:none;cursor:pointer;white-space:nowrap;transition:all .15s ease}
.nav-feedback-btn:hover{background:var(--surface-2,#f1f3f5);color:var(--accent,#3157d5);border-color:#cbd5e1}
.nav-feedback-btn svg{width:15px;height:15px;fill:none;stroke:currentColor;stroke-width:2;stroke-linecap:round;stroke-linejoin:round;flex-shrink:0}
.nav-feedback-btn + .user-menu-container{margin-left:8px}
@media(min-width:769px) and (max-width:880px){.site-nav{padding:0 14px;gap:2px}.site-nav-brand{font-size:18px;margin-right:6px}.nav-tab{padding:0 8px;font-size:12.5px}.nav-feedback-btn{padding:0 10px;font-size:12px}}
@media(min-width:769px) and (max-width:790px){.nav-feedback-btn span{display:none}.nav-feedback-btn{padding:0;width:36px;border-radius:50%}}
.user-menu-container{margin-left:auto;position:relative}.user-menu-btn{display:inline-flex;align-items:center;justify-content:center;width:36px;height:36px;border-radius:50%;border:1px solid var(--line,#d8dee4);background:var(--surface-2,#f1f3f5);color:var(--muted,#525d6b);cursor:pointer;transition:all .15s}
.user-menu-btn:hover,.user-menu-btn[aria-expanded="true"]{background:#e2e8f0;color:var(--text,#18202a);border-color:#cbd5e1}.user-menu-btn svg{width:18px;height:18px;fill:none;stroke:currentColor;stroke-width:2}
.user-dropdown{position:absolute;top:100%;right:0;margin-top:8px;width:220px;background:var(--surface,#fff);border:1px solid var(--line,#d8dee4);border-radius:8px;box-shadow:var(--shadow,0 8px 28px rgba(20,30,45,.06));opacity:0;visibility:hidden;transform:translateY(-10px);transition:opacity .2s,transform .2s,visibility .2s;z-index:1200;padding:8px 0}
.user-dropdown.show{opacity:1;visibility:visible;transform:translateY(0)}.user-dropdown-item{display:flex;align-items:center;gap:10px;width:100%;padding:10px 16px;border:none;background:none;text-align:left;font:inherit;font-size:13px;font-weight:500;color:var(--text,#18202a);cursor:pointer;text-decoration:none;transition:background .15s}
.user-dropdown-item:hover{background:var(--surface-2,#f1f3f5)}.user-dropdown-item svg{width:16px;height:16px;color:var(--muted,#525d6b)}
.page-header{background:var(--surface,#fff);border-bottom:1px solid var(--line,#d8dee4);padding:34px 28px 28px;text-align:center}.page-header-inner{max-width:1280px;margin:auto}
.page-header h1{font-size:clamp(24px,3vw,34px);line-height:1.1;margin:0 0 8px;letter-spacing:-.03em}.page-header p{margin:0;color:var(--muted,#525d6b);font-size:14px;line-height:1.5}
.menu-toggle{display:none;background:transparent;border:none;padding:8px;margin-left:-8px;margin-right:8px;cursor:pointer;color:var(--text,#18202a)}.menu-toggle svg{width:24px;height:24px;stroke:currentColor;stroke-width:2;fill:none}
.nav-overlay{display:none;position:fixed;inset:0;background:rgba(0,0,0,.5);z-index:1050;opacity:0;transition:opacity .3s}
.scroll-top-btn{position:fixed;bottom:24px;right:24px;width:44px;height:44px;border-radius:50%;background:var(--accent,#3157d5);color:#fff;border:none;box-shadow:0 4px 12px rgba(49,87,213,.3);cursor:pointer;display:flex;align-items:center;justify-content:center;opacity:0;visibility:hidden;transform:translateY(10px);transition:all .3s ease;z-index:900}
.scroll-top-btn.show{opacity:1;visibility:visible;transform:translateY(0)}.scroll-top-btn:hover{background:#2749b8;transform:translateY(-2px);box-shadow:0 6px 16px rgba(49,87,213,.4)}.scroll-top-btn svg{width:20px;height:20px;fill:none;stroke:currentColor;stroke-width:2.5;stroke-linecap:round;stroke-linejoin:round}
.site-footer{max-width:1280px;margin:0 auto;padding:0 28px 32px;color:var(--muted,#525d6b);font-size:11px;line-height:1.5}
.site-footer .footer-row{display:flex;justify-content:space-between;align-items:center;flex-wrap:wrap;gap:8px}
.site-footer .brand-inline{font-size:11.5px}
@media(max-width:768px){.menu-toggle{display:inline-flex;align-items:center;justify-content:center;position:relative;z-index:1110;border-radius:6px}.menu-toggle:hover{background:var(--surface-2,#f1f3f5)}.site-nav{position:sticky;top:0;z-index:1100;padding:0 16px;justify-content:flex-start}.site-nav-brand{position:absolute;left:50%;transform:translateX(-50%);margin:0;padding:0}.nav-links{position:fixed;top:52px;left:0;bottom:0;width:260px;background:var(--surface,#fff);z-index:1090;box-shadow:2px 0 12px rgba(0,0,0,.1);transform:translateX(-100%);transition:transform .3s ease;display:flex;flex-direction:column;overflow-y:auto;overflow-x:hidden}.nav-links-inner{display:flex;flex-direction:column;min-height:100%;padding:16px 16px 24px;gap:8px}.nav-tab{height:44px;justify-content:flex-start;border-bottom:none;border-radius:8px;padding:0 12px}.nav-tab.active{background:var(--surface-2,#f1f3f5);border:none}.nav-feedback-btn{margin:auto 0 0;width:100%;height:42px;justify-content:flex-start;border-radius:8px;padding:0 12px;background:var(--surface-2,#f1f3f5);color:var(--text,#18202a);border:1px solid var(--line,#d8dee4);font-size:13px;gap:8px}.nav-feedback-btn:hover{background:#e2e8f0;color:var(--accent,#3157d5)}.nav-feedback-btn svg{width:16px;height:16px;color:var(--muted,#525d6b)}.user-menu-container{margin:auto 0 0;width:100%;border-top:1px solid var(--line,#d8dee4);padding-top:16px}.nav-feedback-btn + .user-menu-container{margin:8px 0 0;border-top:1px solid var(--line,#d8dee4);padding-top:12px}.user-dropdown{position:static;box-shadow:none;border:none;width:100%;opacity:1;visibility:visible;transform:none;padding:0;margin-top:12px}.user-menu-btn{display:none}body.nav-open{overflow:hidden}body.nav-open .nav-links{transform:translateX(0)}.nav-overlay{top:52px}body.nav-open .nav-overlay{display:block;opacity:1}.page-header{padding:28px 16px 24px}.page-header h1{font-size:clamp(22px,6vw,30px)}.site-footer{padding:0 16px 24px}.site-footer .footer-row{flex-direction:column;align-items:flex-start;gap:6px}}