fix(mobile): preserve paired host over local runtime

[riverpilot]

Summary

• retain PR feat: launch bundled runtime from Flutter macOS #128's bundled macOS runtime launch
• treat the bundled endpoint as a fallback rather than a development override
• add coverage proving a saved paired host wins over the local runtime

Supersedes #128.

Screenshots

No visual change.

Testing

• GitHub Flutter Apple and verify checks required; Flutter is unavailable on the local workstation

Risk

• Endpoint precedence changes only when a bundled fallback is configured.

Notes

• Rebased on current main.

AI Review Report

• Addresses the maintainer-requested endpoint-precedence blocker on feat: launch bundled runtime from Flutter macOS #128.

Security Audit

• Preserves explicit paired-host selection; no credential or transport changes.

[iarbpairs]

Architecture review: the bundled Local runtime direction and paired-host precedence are correct, but merge is intentionally held until Gate 1 freezes the common Local/Hub endpoint-profile and provider contracts. After Gate 1, rebase this contribution onto current main, preserve explicit paired-host precedence, and express the bundled fallback through the frozen profile contract. #131 also overlaps apps/flutter/lib/main.dart, so resolve that during the post-Gate-1 rebase.

[riverpilot]

Rebased the replacement onto current main at 551bdfa, including merged #131, and resolved the apps/flutter/lib/main.dart overlap while preserving explicit development endpoint → paired host → bundled local fallback precedence. Focused host-service tests pass (331 tests) and the release consistency check passes. The requested Gate 1 Local/Hub endpoint-profile/provider contract is not present on current main yet—the available Flutter HostProfile contract is still Tailnet-only—so I have intentionally not invented a competing profile abstraction. This PR remains held for the Gate 1 contract freeze; once that lands, I will route the bundled fallback through it and rerun Flutter CI. Packaging tests were 49/50 locally; the sole failure requires prebuilt desktop/web/host artifacts absent from this fresh worktree.

[wolfiesch]

Superseded by the approved Electron standardization direction. This change is coupled to the Flutter macOS runtime path and does not reproduce an independent Electron/React compatibility defect. Any host-policy issue will be recovered separately with a focused reproduction.