Add user authentication with login/register

- Add users and sessions tables to database schema
- Add bcryptjs for password hashing
- Create auth API routes (login, register, logout, me)
- Add AuthProvider context for client-side auth state
- Update all API routes to require authentication and filter by userId
- Create login and register pages
- Add AppShell component for authenticated layout
- Update all pages to use AppShell and show user info
- Each user now has their own private entries and tags

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/AppShell.tsx

@@ -0,0 +1,19 @@
+"use client";
+
+import { useAuth } from "./AuthProvider";
+import { Navigation } from "./Navigation";
+
+export function AppShell({ children }: { children: React.ReactNode }) {
+  const { user } = useAuth();
+
+  if (!user) {
+    return <>{children}</>;
+  }
+
+  return (
+    <>
+      <main className="max-w-lg mx-auto px-4 py-6 pb-24">{children}</main>
+      <Navigation />
+    </>
+  );
+}