Add night mode red overlay for dark-adapted vision

- NightModeProvider context (localStorage persisted) in contexts/NightMode.tsx
- Full-screen fixed red overlay (rgba 160,0,0 @ 55%, mix-blend-mode: multiply) fades in over the entire UI; multiply blend keeps dark backgrounds black while turning all white/bright content deep red
- Desktop: toggle button at the bottom of the sidebar, glows red when active
- Mobile: floating red circle button fixed just above the bottom nav bar

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

frontend/src/styles/global.css

@@ -107,6 +107,7 @@ input:focus, select:focus, textarea:focus {
   /* Sidebar hidden; bottom nav shown */
   .app-sidebar    { display: none !important; }
   .bottom-nav     { display: flex !important; }
+  .night-fab      { display: flex !important; }
 
   /* Main content: zero out PageShell padding; bottom-nav clearance */
   .app-main       { padding: 0 0 68px 0 !important; }