🐛 Ignore Zone.js toString TypeError in uncaught error handler (#8804)

Zone.js (injected by browser extensions such as Angular DevTools) patches
addEventListener by wrapping it and assigning a custom .toString to the
wrapper via Object.defineProperty with writable:false.  When the same
element is processed a second time, the plain assignment in strict mode
(libs.js is built with a "use strict" banner) throws a native TypeError:
"Cannot assign to read only property 'toString' of function '...'".

This error escapes the React tree through the window error/unhandledrejection
events and was surfacing the exception page to users even though Penpot itself
is unaffected.

The fix:
- Extract the private ignorable-exception? helpers from the letfn block into
  top-level defn/defn- forms so the predicate can be reused elsewhere.
- Add the Zone.js toString TypeError to the ignorable-exception? predicate so
  the global uncaught-error handler silently suppresses it.
- The React error boundary is intentionally left unchanged: anything that
  reaches it has executed inside React's reconciler and must not be ignored.

frontend/src/app/main/errors.cljs

@@ -390,7 +390,16 @@
                   ;; RxJS unsubscription / take-until chain).  These are
                   ;; handled gracefully inside app.util.http/fetch and must NOT
                   ;; be surfaced as application errors.
-                  (= (.-name ^js cause) "AbortError"))))
+                  (= (.-name ^js cause) "AbortError")
+                  ;; Zone.js (injected by browser extensions such as Angular
+                  ;; DevTools) wraps event listeners and assigns a custom
+                  ;; .toString to its wrapper functions using
+                  ;; Object.defineProperty.  When the wrapper was previously
+                  ;; defined with {writable: false}, a subsequent plain assignment
+                  ;; in strict mode (our libs.js uses "use strict") throws this
+                  ;; TypeError.  This is a known Zone.js / browser-extension
+                  ;; incompatibility and is NOT a Penpot bug.
+                  (str/starts-with? message "Cannot assign to read only property 'toString'"))))
 
           (on-unhandled-error [event]
             (.preventDefault ^js event)