feat: add Formulus–Synkronus version handshake (major-version check and x-formulus-version header)

[Ndacyayisenga-droid]

Description

This PR implements a version handshake mechanism between the Formulus React Native app and the Synkronus Go backend server. Every request from Formulus now carries its version, and Synkronus validates compatibility before processing anything. Incompatible versions are rejected immediately with a clear, user-friendly error.

Breaking Change

This PR introduces a breaking change by design. Any client not sending the x-formulus-version header receives HTTP 426 immediately. This is intentional, it establishes the version protocol from day one of the first release. Old Formulus builds without this header will be rejected.

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How It Works

Every Formulus request includes a version header:

x-formulus-version: 1.0.0

Synkronus middleware intercepts every request and validates it:

Incoming request
      ↓
Header missing?          → 426 Upgrade Required
Header unparseable?      → 426 Upgrade Required
Server version dev/bad?  → 426 Upgrade Required
Major versions differ?   → 426 Upgrade Required
Major versions match?    → ✅ Request proceeds

Every response (success or error) includes:

X-Synkronus-Version: 1.0.0

This lets the client display exactly which server version it's talking to in error messages.

Version Compatibility Rules

Formulus	Synkronus	Compatible?
1.0.0	1.0.0	✅ Yes
1.3.2	1.5.0	✅ Yes — same major
2.0.0	1.0.0	❌ No — major differs
1.0.0	2.0.0	❌ No — major differs

Known Limitation is iOS Version Sync

syncNativeVersion.js currently only syncs to Android's build.gradle. iOS version fields in project.pbxproj are not updated automatically. If building for iOS, manually ensure MARKETING_VERSION in Xcode matches package.json. iOS automation is tracked as a future enhancement.

[r0ssing]

Looks good!!