feat.: add BLE security via 4-digit auth code

[Rampage270906]

Closes #72
The badge's BT-PAIRING mode accepted bitmap transfers from any nearby device i.e anyone running the app within BLE range could overwrite the display without the owner's knowledge or consent. Added an application-layer authentication gate on top of the existing BLE connection flow. When the badge enters BT-PAIRING mode, it generates a random 4-digit code and displays it on the LED matrix. Any device attempting to transfer data must first send the correct code before the badge will accept a 'wang' transfer and without it, all writes are rejected.

Reviewer's Guide:
fossasia/badgemagic-app#1783 (comment)
This feature is dependent on the app and requires the above PR to be merged in the badgmagic-app repo

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[weberval]

This is a breaking change!
Not only the flutter app will be affected by this, but also the Rust, Go and Python libraries.

To lower the impact and enable other developers to incorporate the changes, I would like to see some documentation about the new protocol extension.

Also, to be backward compatible to the proprietary firmware, we would need a way to keep insecure pairing. Maybe by pressing a button in pairing mode?

[Rampage270906]

Will look into this and add the following at the earliest.

• protocol.md documenting the auth packet format and lifecycle
• A KEY4 bypass in BT-PAIRING mode that allows legacy firmware users to connect without PIN
• Security mode defaults to OFF so existing users/libraries are unaffected until they explicitly enable it