Allow Decodex App fallback server dogfood

[yvette-carlisle]

Summary

• start the bundled Decodex App fallback server with --allow-unverified-codex
• keep the app fallback command documented with the same fail-closed boundary as the CLI override
• add a Swift test that locks the fallback server arguments
• remove one Swift style-gate false-positive by rewriting !refreshTokenPresent as refreshTokenPresent == false

Verification

• swift test --package-path apps/decodex-app
• cargo vstyle curate --language swift --workspace
• cargo make test-decodex-app-stage
• git diff --check
• semantic drift audit: pass; README fallback command matches DecodexServerBridge.bundledServerArguments and the new test