fix: mobile UX overhaul — collapsible filters, settings nav, responsive layout

- Add Settings link to desktop nav bar for admin users
- Rewrite Settings page from table layout to responsive card list (fixes unusable mobile state)
- Filter bar (Dashboard + Comparison): collapsible panel on mobile via display:contents trick; stacked full-width dropdowns replace horizontal scroll
- Active filter count badge shown in collapsed filter header
- AltMultiSelect dropdowns go full-width on mobile to prevent viewport overflow
- Chart control separators hidden on mobile to avoid crowding
- Metric grid: 2-col at ≤700px, 1-col at ≤480px
- Comparison period cards: smaller font and tighter padding at ≤680px
- Page shell padding reduced on mobile (48px→20px top, 24px→16px sides)
- Settings page gets correct 80px bottom padding for mobile nav

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

src/App.tsx

@@ -237,6 +237,14 @@ function App() {
                 </svg>
                 {t('nav.comparison')}
               </NavLink>
+              {userRole === 'admin' && (
+                <NavLink to="/settings">
+                  <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+                    <circle cx="12" cy="12" r="3"/><path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1-2.83 2.83l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-4 0v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83-2.83l.06-.06A1.65 1.65 0 0 0 4.68 15a1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1 0-4h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 2.83-2.83l.06.06A1.65 1.65 0 0 0 9 4.68a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 4 0v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 2.83l-.06.06A1.65 1.65 0 0 0 19.4 9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 0 4h-.09a1.65 1.65 0 0 0-1.51 1z"/>
+                  </svg>
+                  {t('nav.settings')}
+                </NavLink>
+              )}
               <span className="nav-sep" aria-hidden="true" />
               {isOffline && (
                 <span className="offline-badge" title={cacheInfo ? `Cached: ${new Date(cacheInfo.timestamp || '').toLocaleString()}` : ''}>