[FEAT]: Platform agnostic backup & restore file format

[jamesarich]

We need to define and implement a single, consistent file format for backing up and restoring Meshtastic app data across platforms and environments. Currently, backup and restore mechanisms use platform centric backup methods and file formats and ad-hoc serialization strategies, which complicates interoperability, versioning, and user experience.

The unified format should:

• Support structured, versioned data that can evolve over time (e.g., through schema versioning or metadata blocks).
• Allow partial and full restores (e.g., restoring only certain datasets or settings).
• Be cross-compatible between environments (e.g., web, desktop, mobile).
• Include integrity checks (checksum or hash validation) to prevent corrupted restores or invalid backups from being restored (optional).
• Be easy to parse and validate with a stable schema (e.g., JSON, YAML, or a binary structure).

Goals

• Simplify backup/restore implementation and maintenance.
• Enable seamless migration of user data across devices (IOS, Android, Web).

Sub-tasks

• Create Android alignment issue.
• Create iOS alignment issue.
• Create Web alignment issue.

[garthvh]

The transaction system in the firmware is too unstable at this point to really consider this, needs over 100 tests in the firmware repo first so this feature does not break every time config changes are made.