feat: dark mode support, markdown descriptions, inline editing on TaskPage

- Full dark mode across TaskPage (header, cards, sidebar, forms)
- Task descriptions rendered as markdown (ReactMarkdown + remark-gfm)
- Inline description editing with markdown preview
- Inline title editing (click to edit)
- Theme system (useTheme hook with light/dark/system toggle)
- Dark mode classes across remaining components

frontend/src/App.tsx

@@ -3,6 +3,7 @@ import { BrowserRouter, Routes, Route, Navigate } from "react-router-dom";
 import { DashboardLayout } from "./components/DashboardLayout";
 import { LoginPage } from "./components/LoginPage";
 import { ToastProvider } from "./components/Toast";
+import { ThemeProvider } from "./hooks/useTheme";
 import { useSession } from "./lib/auth-client";
 
 // Lazy-loaded pages for code splitting
@@ -50,20 +51,26 @@ function App() {
 
   if (session.isPending) {
     return (
-      <div className="min-h-screen bg-gray-50 flex items-center justify-center">
+      <div className="min-h-screen bg-gray-50 dark:bg-gray-950 flex items-center justify-center">
         <div className="text-gray-400">Loading...</div>
       </div>
     );
   }
 
   if (!session.data) {
-    return <LoginPage onSuccess={() => window.location.reload()} />;
+    return (
+      <ThemeProvider>
+        <LoginPage onSuccess={() => window.location.reload()} />
+      </ThemeProvider>
+    );
   }
 
   return (
-    <ToastProvider>
-      <AuthenticatedApp />
-    </ToastProvider>
+    <ThemeProvider>
+      <ToastProvider>
+        <AuthenticatedApp />
+      </ToastProvider>
+    </ThemeProvider>
   );
 }