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/App.tsx

@@ -7,10 +7,26 @@ import Stats from './pages/Stats';
 import Settings from './pages/Settings';
 import Gallery from './pages/Gallery';
 import SolarSystem from './pages/SolarSystem';
+import { NightModeProvider, useNightMode } from './contexts/NightMode';
 
-export default function App() {
+function NightOverlay() {
+  const { on } = useNightMode();
   return (
-    <BrowserRouter>
+    <div style={{
+      position: 'fixed', inset: 0, zIndex: 9999,
+      background: 'rgba(160, 0, 0, 0.55)',
+      mixBlendMode: 'multiply',
+      pointerEvents: 'none',
+      opacity: on ? 1 : 0,
+      transition: 'opacity 0.4s ease',
+    }} />
+  );
+}
+
+function AppInner() {
+  return (
+    <>
+      <NightOverlay />
       <PageShell>
         <Routes>
           <Route path="/" element={<Navigate to="/dashboard" replace />} />
@@ -24,6 +40,16 @@ export default function App() {
           <Route path="/settings" element={<Settings />} />
         </Routes>
       </PageShell>
-    </BrowserRouter>
+    </>
+  );
+}
+
+export default function App() {
+  return (
+    <NightModeProvider>
+      <BrowserRouter>
+        <AppInner />
+      </BrowserRouter>
+    </NightModeProvider>
   );
 }

frontend/src/components/layout/Sidebar.tsx

@@ -3,6 +3,7 @@ import { useTonight } from '../../hooks/useTonight';
 import { useWeather, useForecast } from '../../hooks/useWeather';
 import MoonPhaseIcon from '../sky/MoonPhaseIcon';
 import GoNogo from '../weather/GoNogo';
+import { useNightMode } from '../../contexts/NightMode';
 
 const SEEING_LABELS: Record<number, string> = {
   1: '0.5″', 2: '0.75″', 3: '1.0″', 4: '1.25″',
@@ -31,7 +32,35 @@ function fmtTime(utc?: string): string {
 }
 
 export function BottomNav() {
+  const { on, toggle } = useNightMode();
   return (
+    <>
+      {/* Night mode floating toggle — sits just above bottom nav */}
+      <button
+        onClick={toggle}
+        title={on ? 'Exit night mode' : 'Night mode'}
+        style={{
+          position: 'fixed',
+          bottom: 66,
+          right: 14,
+          zIndex: 210,
+          width: 36, height: 36,
+          borderRadius: '50%',
+          background: on ? 'rgba(160,0,0,0.85)' : 'var(--bg-panel)',
+          border: `1px solid ${on ? '#800000' : 'var(--border)'}`,
+          color: on ? '#ff6666' : 'var(--text-lo)',
+          fontSize: 16,
+          display: 'none', // shown by .bottom-nav display:flex breakpoint via CSS class
+          alignItems: 'center',
+          justifyContent: 'center',
+          cursor: 'pointer',
+          boxShadow: on ? '0 0 12px rgba(160,0,0,0.5)' : 'none',
+          transition: 'all 0.3s',
+        }}
+        className="night-fab"
+      >
+        🔴
+      </button>
       <nav className="bottom-nav">
         {navItems.map(item => (
           <NavLink
@@ -44,6 +73,7 @@ export function BottomNav() {
           </NavLink>
         ))}
       </nav>
+    </>
   );
 }
 
@@ -51,6 +81,7 @@ export default function Sidebar() {
   const { data: tonight } = useTonight();
   const { data: weather } = useWeather();
   const { data: forecast } = useForecast();
+  const { on: nightOn, toggle: nightToggle } = useNightMode();
 
   const slot = (forecast as { dataseries?: { seeing?: number; transparency?: number; cloudcover?: number }[] })?.dataseries?.[0];
 
@@ -147,6 +178,25 @@ export default function Sidebar() {
         </table>
       </div>
 
+      {/* Night mode toggle */}
+      <div style={{ borderTop: '1px solid var(--border)', padding: '10px 16px' }}>
+        <button
+          onClick={nightToggle}
+          style={{
+            width: '100%', display: 'flex', alignItems: 'center', gap: 10,
+            background: nightOn ? 'rgba(160,0,0,0.2)' : 'transparent',
+            border: `1px solid ${nightOn ? '#600000' : 'var(--border)'}`,
+            borderRadius: 4, padding: '7px 10px', cursor: 'pointer',
+            color: nightOn ? '#ff4444' : 'var(--text-lo)',
+            fontFamily: 'var(--font-mono)', fontSize: 11,
+            letterSpacing: '0.06em', transition: 'all 0.2s',
+          }}
+        >
+          <span style={{ fontSize: 13 }}>🔴</span>
+          {nightOn ? 'Exit Night Mode' : 'Night Mode'}
+        </button>
+      </div>
+
       {/* Conditions widget */}
       <div style={{ borderTop: '1px solid var(--border)', padding: '12px 16px' }}>
         <div style={{ fontSize: 10, fontFamily: 'var(--font-mono)', color: 'var(--text-lo)', letterSpacing: '0.1em', marginBottom: 8, textTransform: 'uppercase' }}>

frontend/src/contexts/NightMode.tsx

@@ -0,0 +1,18 @@
+import { createContext, useContext, useState, type ReactNode } from 'react';
+
+interface NightModeCtx { on: boolean; toggle: () => void; }
+const Ctx = createContext<NightModeCtx>({ on: false, toggle: () => {} });
+
+export function NightModeProvider({ children }: { children: ReactNode }) {
+  const [on, setOn] = useState(() => localStorage.getItem('astronome_night') === '1');
+
+  const toggle = () => setOn(v => {
+    const next = !v;
+    localStorage.setItem('astronome_night', next ? '1' : '0');
+    return next;
+  });
+
+  return <Ctx.Provider value={{ on, toggle }}>{children}</Ctx.Provider>;
+}
+
+export const useNightMode = () => useContext(Ctx);

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; }