Add self-service account deletion

Settings → Delete account removes the signed-in account and everything it
owns. DELETE /api/me re-checks the password (guarding an unattended session),
then wipes the user's events, config, sessions and user row in one transaction
and removes their photos/<user_id>/ directory. The client clears the account's
local cache and returns to the login screen.

Bumps the service-worker cache so clients pick up the new UI.

Verified: wrong password is rejected (401, data intact); correct password
returns 204, invalidates the session, drops all rows to zero and removes the
photo dir; the email can be re-registered afterwards. Confirmed end to end in
a headless-browser run of the Settings → delete flow.

src/app.js

@@ -1522,6 +1522,50 @@
     settingsDialog.close();
   });
 
+  // ---------- delete account ----------
+  const deleteAccountDialog   = document.getElementById("delete-account-dialog");
+  const deleteAccountPassword = document.getElementById("delete-account-password");
+  const deleteAccountError    = document.getElementById("delete-account-error");
+  const deleteAccountConfirm  = document.getElementById("delete-account-confirm");
+
+  document.getElementById("delete-account-btn").addEventListener("click", () => {
+    settingsDialog.close();
+    deleteAccountPassword.value = "";
+    deleteAccountError.hidden = true;
+    deleteAccountDialog.showModal();
+    setTimeout(() => deleteAccountPassword.focus(), 50);
+  });
+
+  deleteAccountConfirm.addEventListener("click", async () => {
+    deleteAccountError.hidden = true;
+    deleteAccountConfirm.disabled = true;
+    try {
+      const res = await fetch("api/me", {
+        method: "DELETE",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ password: deleteAccountPassword.value }),
+      });
+      if (!res.ok) {
+        const msg = (await res.text()).trim();
+        throw new Error(res.status === 401 ? "Incorrect password" : (msg || `HTTP ${res.status}`));
+      }
+      // Account is gone server-side. Wipe this user's local cache before the
+      // reload drops us back on the login screen.
+      try {
+        localStorage.removeItem(eventsKey());
+        localStorage.removeItem(configKey());
+      } catch { /* ignore */ }
+      clearUser();
+      deleteAccountDialog.close();
+      location.reload();
+    } catch (err) {
+      deleteAccountError.textContent = err.message || "Something went wrong";
+      deleteAccountError.hidden = false;
+    } finally {
+      deleteAccountConfirm.disabled = false;
+    }
+  });
+
   // ---------- wiring ----------
   document.querySelectorAll("button.action").forEach(btn => {
     btn.addEventListener("click", () => openNoteDialog(btn.dataset.type));