fix(ipc): guard webChannelTransport.onmessage before invoking

[OcheOps]

Problem

The preload script (src/app/ipc/preload.js) unconditionally calls window.qt.webChannelTransport.onmessage(message) inside its message listener, but the transport shim in the same file only assigns .send. .onmessage is expected to be installed later by the hosted stremio-web app.

If a message arrives from the shell before (or without) the web app assigning .onmessage, the listener throws:

TypeError: undefined is not a function

which the Rust side surfaces at startup as:

ERROR stremio_linux_shell::app::webview: Failed to send message: TypeError: undefined is not a function

This is one of the errors reported in #68 / #73 / stremio-bugs#2646 and appears in every session on affected setups (Fedora 44 + AMD + GNOME/Wayland, but not limited to that combo — same log line appears in CachyOS + AMD reports).

Fix

Guard the call with a typeof check. When the transport hasn't been fully initialized, the listener is now a no-op instead of throwing. Once stremio-web installs its onmessage handler, behavior is unchanged.

 window.ipc.addEventListener('message', (message) => {
-    window.qt.webChannelTransport.onmessage(message);
+    if (typeof window.qt.webChannelTransport.onmessage === 'function') {
+        window.qt.webChannelTransport.onmessage(message);
+    }
 });

Notes

• Scope is intentionally minimal — silences the spurious error only. Does not address the mpv VO init failure or the "audio keeps playing after quit" behavior reported in the same tickets; those are separate.
• No regression once .onmessage is assigned: typeof check passes and the message is delivered as before.
• Messages that would previously have thrown are now dropped silently. This matches the intent — if there's no handler to receive them, throwing was never useful.

Refs #68, #73.

[tymmesyde]

Hi, is this PR supposed to make this error message disappear?
ERROR stremio_linux_shell::app::webview: Failed to send message: TypeError: undefined is not a function