Wire Sentry into native release builds

[TaprootFreak]

Summary

• inject the repository SENTRY_DSN into Android and iOS store builds with an explicit production/internal environment
• keep compile-time values out of Fastlane process arguments via a mode-0600 temporary define file and fail release builds when the DSN is missing
• upload Android split debug info, Dart obfuscation maps, and iOS archive symbols to the self-hosted Sentry project
• identify uploads with the native release name swiss.realunit.app@<version>+<build> and matching distribution

The Sentry project realunitchapp and the repository secrets SENTRY_DSN and SENTRY_AUTH_TOKEN have been provisioned. Runtime reporting activates together with #878. Related to DFXServer/server#958.

Validation

• Ruby syntax checks for both Fastfiles
• workflow YAML parse and git diff --check
• flutter analyze --no-fatal-infos
• non-golden Flutter suite: 4,687 tests passed; the 16 subprocess cases initially failed only because local dart was absent from PATH and all 16 passed when rerun with the Flutter SDK path
• DFX PR precheck: 3 rounds, final conformity and logic reviews both at 0 findings

Follow-up verification

The first tagged native release should confirm the uploaded debug files in Sentry and symbolicate one controlled test event.

[TaprootFreak]

Final review completed in 1 pass with 0 findings. No changes were required during this pass; the pre-publication review had already corrected iOS build metadata propagation and added complete native/Dart symbol upload coverage. Full CI is running against the final commit and remains the gate before marking this PR ready for review.