feat: refactor theme management and remove next-themes dependency

- Replaced the next-themes package with a custom ThemeProvider component for enhanced theme management.
- Implemented local storage handling for theme persistence and system theme detection.
- Updated the Toaster component to utilize the new theme context.
- Removed unused next-themes references from package.json and package-lock.json.

components/app-sidebar.tsx

@@ -209,10 +209,10 @@ export function AppSidebar({ ...props }: React.ComponentProps<typeof Sidebar>) {
         title: it.title,
         url: it.url,
         icon: it.icon,
-        badge: badgeFor(it.url),
+        badge: mounted ? badgeFor(it.url) : undefined,
       })),
     }))
-  }, [mode, backendStatus, liveCounts, mockBadges, evo.enabled, evo.snapshot])
+  }, [mounted, mode, backendStatus, liveCounts, mockBadges, evo.enabled, evo.snapshot])
 
   const isDark = (resolvedTheme ?? "dark") === "dark"