Fix Chrome MV3 background suspension using offscreen document

[sayang1080]

Problem

Chrome Manifest V3 suspends background service workers, which breaks the
existing polling-based submit detection after the extension becomes idle.

Solution

• Move polling logic to an offscreen document (MV3-compatible)
• Keep background script event-based
• Preserve Firefox behavior by falling back to background polling
• No user-visible behavior changes

Notes

• Offscreen documents are only created on Chrome/Brave where required.
• Firefox does not use offscreen and continues to work as before.
• This avoids relying on content-script keep-alive ports.

Testing

• Tested on Chrome/Brave with unpacked extension
• Firefox path preserved via conditional fallback

[adervark]

check last PR