Fix org-caldav: @ encoding in UIDs + nil-field event guard

Two patches in org-caldav use-package! config:

Fix #1: org-caldav-get-event encodes @ as %40 via url-hexify-string.
Baikal stores files with literal @ in filename (e.g. ...@google.com.ics).
Result: HTTP 404 after 5 retries for UIDs with @google.com suffix.
Advice: cl-letf temporarily replaces url-hexify-string to decode %40 back to @.

Fix #2: Some CalDAV events have nil SUMMARY/DTSTART. icalendar parser
returns nil, org-caldav passes to string-match -> Wrong type argument: stringp nil.
Advice: condition-case in org-caldav-update-events-in-org catches the error,
logs it to *org-caldav-debug* buffer, and allows sync state to be saved.

config.el

@@ -1775,6 +1775,29 @@ current frame."
   (setq org-caldav-delete-org-entries 'never)
   (setq org-caldav-delete-calendar-entries 'never)
 
+  ;; Fix #1: Baikal stores .ics files with literal @ in filename.
+  ;; org-caldav-get-event uses url-hexify-string which encodes @ as %40 → HTTP 404.
+  ;; UIDs like ...@google.com are stored on disk as literal @, so we must not encode it.
+  (defadvice org-caldav-get-event (around fix-at-encoding-in-uid
+                                         (uid &optional with-headers) activate)
+    "Preserve @ in UID path for Baikal CalDAV compatibility."
+    (cl-letf* ((orig-hexify (symbol-function 'url-hexify-string))
+               ((symbol-function 'url-hexify-string)
+                (lambda (str)
+                  (replace-regexp-in-string "%40" "@" (funcall orig-hexify str)))))
+      ad-do-it))
+
+  ;; Fix #2: Some CalDAV events have nil SUMMARY or missing fields, causing
+  ;; Wrong type argument: stringp, nil inside org-caldav-update-events-in-org.
+  ;; This advice catches the error and logs it so sync continues to save state.
+  (defadvice org-caldav-update-events-in-org (around skip-nil-field-events activate)
+    "Catch nil-field errors from malformed events; log and continue."
+    (condition-case err
+        ad-do-it
+      (wrong-type-argument
+       (message "org-caldav: skipped event with nil field: %S" err)
+       (org-caldav-debug-print 1 (format "Skipped nil-field event: %S" err)))))
+
   (defun my/org-caldav-sync ()
     "Sync 3 CalDAV kalendářů:
   1. Osobni-Suky (default): stahuj vše → caldav-suky.org, nahraj jen calendar_outbox.org