feat: day/night mode toggle with theme context

Add ThemeContext (dark/light, persisted to localStorage, applied as
data-theme on <html>). Theme toggle is a sun/moon flip switch at the
right end of the dashboard info bar.

CSS overrides in index.css target every hardcoded Tailwind arbitrary
colour class used across all pages via [data-theme="light"] selectors,
so the entire panel switches without touching individual page files.

Light palette: page #f0f4f8, cards/sidebar #ffffff, hover #f1f5f9,
borders #e2e8f0, text #0f172a. Chart tooltip is made theme-aware via
inline-style interpolation since Tailwind CSS overrides cannot reach
Recharts inline styles.

nixserver/src/App.tsx

@@ -1,6 +1,7 @@
 import React from 'react'
 import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom'
 import { AuthProvider, useAuth } from './contexts/AuthContext'
+import { ThemeProvider } from './contexts/ThemeContext'
 import Layout from './components/Layout'
 import Login from './pages/Login'
 import Dashboard from './pages/Dashboard'
@@ -233,9 +234,11 @@ function PageLoader() {
 export default function App() {
   return (
     <BrowserRouter>
-      <AuthProvider>
-        <AppRoutes />
-      </AuthProvider>
+      <ThemeProvider>
+        <AuthProvider>
+          <AppRoutes />
+        </AuthProvider>
+      </ThemeProvider>
     </BrowserRouter>
   )
 }