build: support reproducible app bundles

[Betafer]

Summary

• allow a fixed Receiver version and build number through APP_VERSION / APP_BUILD
• allow a fixed Sender build number through TB_SENDER_BUILD
• let the Sender packaging script build either Debug or Release through CONFIGURATION
• keep the generated Xcode project and its source project.yml aligned

Why

The setup assistant and release packaging need deterministic app identities. Timestamp-only builds make it difficult to verify that the tested bundle is the one being installed, while a Debug-only packaging path cannot validate a release candidate.

Validation

• shell syntax checks pass for both packaging scripts
• Receiver bundle built with version 3.5.1 and build 20260821000101; both values were verified in its Info.plist and executable
• Sender Release bundle built with build 20260821000102; the value was verified in the executable
• Sender output is a signed universal Mach-O bundle for arm64 and x86_64

This PR changes only four build-definition lines and their generated-project counterpart.