chore: migrate encrypt to cryptography and upgrade objectbox

[cameronaaron]

Summary

• Migrate from archived encrypt package to actively maintained cryptography package
• Upgrade objectbox from 4.0.3 to ^5.0.3

Changes

• encrypt: ^5.0.3 → cryptography: ^2.7.0
• objectbox: 4.0.3 → objectbox: ^5.0.3
• Updated crypto functions to async (cryptography package doesn't support sync operations)
• Added ServerPayload.fromJsonAsync() static factory for encrypted payloads
• Updated all call sites in action_handler.dart and method_channel_service.dart

Why

The encrypt package is archived and no longer maintained. The cryptography package is actively maintained and provides modern encryption APIs.

Testing

• flutter analyze: 0 errors (451 pre-existing warnings/infos)
• flutter pub get: Dependencies resolve cleanly

[jjoelj]

https://github.com/BlueBubblesApp/bluebubbles-app/pull/2959 switches to encrypter_plus and upgrades objectbox

[cameronaaron]

#2959 switches to encrypter_plus and upgrades objectbox

Oh let me know when this is done there is other modernization I was working on that was blocked by the older encryption lib