Implement rate limiting and enhance authentication flow

- Added rate limiting configuration for authentication endpoints, allowing customizable request limits via environment variables.
- Updated authentication flow to utilize HttpRequest for cookie management, ensuring secure handling of refresh tokens.
- Introduced a new endpoint to retrieve user subscription details.
- Enhanced the handling of Telegram bot token validation to prevent errors with empty tokens.
- Updated the application to serialize enums as strings for better documentation and compatibility with TypeScript.
- Improved test coverage for new features and adjustments in command handlers.

frontend/src/theme/ThemeProvider.tsx

@@ -0,0 +1,51 @@
+import { createContext, useContext, useEffect, useState, type ReactNode } from 'react'
+
+export type Theme = 'light' | 'dark' | 'system'
+
+type ThemeContextValue = {
+  theme: Theme
+  setTheme: (theme: Theme) => void
+}
+
+const STORAGE_KEY = 'pnv-theme'
+const ThemeContext = createContext<ThemeContextValue | undefined>(undefined)
+
+function resolve(theme: Theme): 'light' | 'dark' {
+  if (theme === 'system') {
+    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+  }
+  return theme
+}
+
+function applyTheme(theme: Theme) {
+  const root = document.documentElement
+  root.classList.toggle('dark', resolve(theme) === 'dark')
+}
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>(
+    () => (localStorage.getItem(STORAGE_KEY) as Theme | null) ?? 'system',
+  )
+
+  useEffect(() => {
+    applyTheme(theme)
+    if (theme !== 'system') return
+    const media = window.matchMedia('(prefers-color-scheme: dark)')
+    const onChange = () => applyTheme('system')
+    media.addEventListener('change', onChange)
+    return () => media.removeEventListener('change', onChange)
+  }, [theme])
+
+  const setTheme = (next: Theme) => {
+    localStorage.setItem(STORAGE_KEY, next)
+    setThemeState(next)
+  }
+
+  return <ThemeContext value={{ theme, setTheme }}>{children}</ThemeContext>
+}
+
+export function useTheme(): ThemeContextValue {
+  const ctx = useContext(ThemeContext)
+  if (!ctx) throw new Error('useTheme must be used within ThemeProvider')
+  return ctx
+}