feat: initial implementation of Oriana uptime monitor

Implements a full-stack uptime monitoring application with:
- HTTPS, HTTPS keyword, HTTPS status, port, and MongoDB Atlas monitor types
- Cron-based monitoring engine with webhook notifications on status changes
- Discord OAuth2 admin panel (single-owner)
- Public status page with category grouping and failure reason display
- Admin dashboard with sortable monitors table and detailed failure info
- SQLite persistence with migration support

client/src/pages/Login.tsx

@@ -0,0 +1,81 @@
+import { useEffect } from "react";
+import { useNavigate } from "react-router-dom";
+import { useAuth } from "../hooks/useAuth.js";
+
+export const Login = (): React.ReactElement => {
+  const { user, loading } = useAuth();
+  const navigate = useNavigate();
+
+  useEffect(() => {
+    if (!loading && user) {
+      navigate("/admin");
+    }
+  }, [user, loading, navigate]);
+
+  const error = new URLSearchParams(window.location.search).get("error");
+
+  return (
+    <div
+      style={{
+        display: "flex",
+        flexDirection: "column",
+        alignItems: "center",
+        justifyContent: "center",
+        minHeight: "100vh",
+      }}
+    >
+      <div
+        style={{
+          background: "#44275a",
+          border: "1px solid #a8577e",
+          borderRadius: 15,
+          padding: 40,
+          textAlign: "center",
+          maxWidth: 360,
+          width: "100%",
+        }}
+      >
+        <h1 style={{ margin: "0 0 8px", fontSize: 28, color: "#d4a5c7" }}>Oriana</h1>
+        <p style={{ color: "#c0c0c0", margin: "0 0 24px", fontSize: 14 }}>
+          Admin access via Discord
+        </p>
+        {error === "unauthorised" && (
+          <div
+            style={{
+              background: "#0a0009",
+              border: "1px solid #f87171",
+              borderRadius: 8,
+              padding: "10px 14px",
+              color: "#f87171",
+              fontSize: 13,
+              marginBottom: 16,
+            }}
+          >
+            Your Discord account is not authorised to access this dashboard.
+          </div>
+        )}
+        <a
+          href="/auth/discord"
+          style={{
+            display: "inline-block",
+            background: "#5865f2",
+            color: "#fff",
+            padding: "12px 24px",
+            borderRadius: 8,
+            textDecoration: "none",
+            fontWeight: 600,
+            fontSize: 15,
+            transition: "opacity 0.3s ease",
+          }}
+        >
+          Sign in with Discord
+        </a>
+        <div style={{ marginTop: 20 }}>
+          <a href="/" style={{ fontSize: 13, color: "#d4a5c7" }}>
+            ← Back to status page
+          </a>
+        </div>
+      </div>
+    </div>
+  );
+};