🐛 Add timeout to plugin manifest fetch (#11120)

The fetch-manifest function previously had no timeout, causing the
plugin installation flow to hang indefinitely if the server accepted
the connection but never completed the response.

Added a 15-second timeout using rx/timeout to abort the request
automatically.

Closes #11119

AI-assisted-by: qwen3.7-plus

frontend/src/app/main/data/plugins.cljs

@@ -39,6 +39,7 @@
                     :uri plugin-url
                     :omit-default-headers true
                     :response-type :json})
+       (rx/timeout 15000)
        (rx/map :body)
        (rx/map #(preg/parse-manifest plugin-url %))))