Add Danger Zone section to settings page with delete account dialog

Adds a destructively-styled Danger Zone card to src/app/app/settings/page.tsx.
Clicking "Delete Account" opens a shadcn/ui Dialog that warns the user the
action is irreversible, requires typing "DELETE" to enable the confirm button,
calls DELETE /api/auth/account on confirmation, then signs the user out and
redirects to "/". Marks task 12.3 complete in tasks.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/app/settings/page.tsx

@@ -1,12 +1,20 @@
 "use client";
 
 import { useState, useEffect } from "react";
-import { useSession } from "next-auth/react";
+import { useSession, signOut } from "next-auth/react";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { Card, CardContent, CardHeader, CardTitle, CardDescription } from "@/components/ui/card";
-import { CheckCircle, AlertCircle } from "lucide-react";
+import {
+  Dialog,
+  DialogContent,
+  DialogDescription,
+  DialogFooter,
+  DialogHeader,
+  DialogTitle,
+} from "@/components/ui/dialog";
+import { CheckCircle, AlertCircle, Trash2 } from "lucide-react";
 
 export default function SettingsPage() {
   const { data: session, update } = useSession();
@@ -20,6 +28,12 @@ export default function SettingsPage() {
   // User provider state (fetched from API)
   const [userProvider, setUserProvider] = useState<string | null>(null);
 
+  // Danger Zone — delete account state
+  const [deleteDialogOpen, setDeleteDialogOpen] = useState(false);
+  const [deleteConfirmText, setDeleteConfirmText] = useState("");
+  const [isDeletingAccount, setIsDeletingAccount] = useState(false);
+  const [deleteError, setDeleteError] = useState<string | null>(null);
+
   // Security / change-password state
   const [currentPassword, setCurrentPassword] = useState("");
   const [newPassword, setNewPassword] = useState("");
@@ -120,6 +134,25 @@ export default function SettingsPage() {
     }
   }
 
+  async function handleDeleteAccount() {
+    setDeleteError(null);
+    setIsDeletingAccount(true);
+    try {
+      const response = await fetch("/api/auth/account", { method: "DELETE" });
+      if (!response.ok) {
+        const data = await response.json();
+        setDeleteError(data.error ?? "Failed to delete account.");
+        return;
+      }
+      // Account deleted — sign out and redirect to home
+      await signOut({ redirectTo: "/" });
+    } catch {
+      setDeleteError("An unexpected error occurred. Please try again.");
+    } finally {
+      setIsDeletingAccount(false);
+    }
+  }
+
   const email = session?.user?.email ?? "";
   const isCredentialsUser = userProvider === "credentials";
   const isOAuthUser = userProvider !== null && userProvider !== "credentials";
@@ -198,6 +231,7 @@ export default function SettingsPage() {
         </CardContent>
       </Card>
 
+      {/* ------------------------------------------------------------------ */}
       {/* Security section */}
       {userProvider !== null && (
         <Card>
@@ -318,6 +352,100 @@ export default function SettingsPage() {
           </CardContent>
         </Card>
       )}
+      {/* Danger Zone section */}
+      <Card className="border-destructive/40">
+        <CardHeader>
+          <CardTitle className="text-base font-mono text-destructive">Danger Zone</CardTitle>
+          <CardDescription className="text-xs">
+            Irreversible actions that affect your account and all your data.
+          </CardDescription>
+        </CardHeader>
+        <CardContent>
+          <div className="flex items-start justify-between gap-4 rounded-md border border-destructive/30 bg-destructive/5 p-4">
+            <div>
+              <p className="text-sm font-mono font-medium">Delete Account</p>
+              <p className="text-xs text-muted-foreground font-mono mt-0.5">
+                Permanently delete your account and all associated data. This cannot be undone.
+              </p>
+            </div>
+            <Button
+              variant="destructive"
+              size="sm"
+              className="font-mono shrink-0"
+              onClick={() => {
+                setDeleteConfirmText("");
+                setDeleteError(null);
+                setDeleteDialogOpen(true);
+              }}
+            >
+              <Trash2 className="h-4 w-4 mr-1.5" />
+              Delete Account
+            </Button>
+          </div>
+        </CardContent>
+      </Card>
+
+      {/* Delete account confirmation dialog */}
+      <Dialog
+        open={deleteDialogOpen}
+        onOpenChange={(open) => {
+          if (!isDeletingAccount) setDeleteDialogOpen(open);
+        }}
+      >
+        <DialogContent className="sm:max-w-md">
+          <DialogHeader>
+            <DialogTitle className="font-mono text-destructive">Delete Account</DialogTitle>
+            <DialogDescription className="font-mono text-sm">
+              This action is <span className="font-semibold text-foreground">irreversible</span>.
+              All your charts, annotations, and data will be permanently deleted.
+            </DialogDescription>
+          </DialogHeader>
+
+          <div className="space-y-4 py-2">
+            {deleteError && (
+              <div className="flex items-center gap-2 rounded-md bg-destructive/10 border border-destructive/20 p-3">
+                <AlertCircle className="h-4 w-4 text-destructive flex-shrink-0" />
+                <p className="text-sm text-destructive font-mono">{deleteError}</p>
+              </div>
+            )}
+
+            <div className="space-y-2">
+              <Label htmlFor="deleteConfirm" className="text-xs font-mono">
+                Type <span className="font-bold text-destructive">DELETE</span> to confirm
+              </Label>
+              <Input
+                id="deleteConfirm"
+                type="text"
+                placeholder="DELETE"
+                value={deleteConfirmText}
+                onChange={(e) => setDeleteConfirmText(e.target.value)}
+                className="font-mono text-sm"
+                disabled={isDeletingAccount}
+                autoComplete="off"
+              />
+            </div>
+          </div>
+
+          <DialogFooter className="gap-2">
+            <Button
+              variant="outline"
+              className="font-mono text-sm"
+              onClick={() => setDeleteDialogOpen(false)}
+              disabled={isDeletingAccount}
+            >
+              Cancel
+            </Button>
+            <Button
+              variant="destructive"
+              className="font-mono text-sm"
+              disabled={deleteConfirmText !== "DELETE" || isDeletingAccount}
+              onClick={handleDeleteAccount}
+            >
+              {isDeletingAccount ? "Deleting…" : "Delete My Account"}
+            </Button>
+          </DialogFooter>
+        </DialogContent>
+      </Dialog>
     </div>
   );
 }