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/style.css

@@ -599,3 +599,18 @@ button.linklike {
   cursor: pointer;
 }
 button.linklike:hover { text-decoration: underline; filter: none; }
+
+/* ---------- delete account ---------- */
+.settings-sep {
+  border: none;
+  border-top: 1px solid var(--border);
+  margin: 18px 0 12px;
+}
+.danger-block { width: 100%; }
+.danger-text {
+  font-size: 0.9rem;
+  color: var(--muted);
+  margin: 0 0 14px;
+  line-height: 1.4;
+}
+.danger-text strong { color: var(--danger); }