feat: add DR Capacity Planner, light/dark mode toggle, and PDF export

- Add zroc-planner UI page with VM selector, journal retention slider (1h-30d),
  WAN compression input, and live bandwidth/journal/mirror storage estimates
- Add CSV and PDF export for planning reports
- Add light/dark mode toggle in TopBar with localStorage persistence
- Wire theme via CSS custom properties for full Tailwind opacity support
- Add Planner route and sidebar entry

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

zroc-ui/src/auth/ThemeContext.jsx

@@ -0,0 +1,29 @@
+// src/auth/ThemeContext.jsx
+import { createContext, useContext, useState, useEffect } from 'react';
+
+const ThemeContext = createContext(null);
+
+export function ThemeProvider({ children }) {
+  const [theme, setTheme] = useState(() =>
+    localStorage.getItem('zroc-theme') || 'dark'
+  );
+
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme);
+    localStorage.setItem('zroc-theme', theme);
+  }, [theme]);
+
+  const toggle = () => setTheme((t) => t === 'dark' ? 'light' : 'dark');
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggle }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const ctx = useContext(ThemeContext);
+  if (!ctx) throw new Error('useTheme must be used within ThemeProvider');
+  return ctx;
+}