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/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);