Fix double initialization with Check-or-Listen and mutex

[google-labs-jules]

Implemented the Check-or-Listen pattern in static/app.js to prevent double initialization of the application.

• Added a isInitialized boolean flag as a mutex in the setup function.
• Updated init to check and set the mutex.
• Updated onMounted to remove the setTimeout fallback and unconditionally adding the event listener. Instead, it now checks window.pywebview first, calling init if present, or adding the pywebviewready listener if not.

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PR created automatically by Jules for task 1504147922092227330 started by @OsakaLOOP

[google-labs-jules]

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