fix: harden auth restoration and server scoping

[mkaltner]

Summary

• harden authentication restoration so valid sessions resume without flashing login, invalid sessions fall back safely, and cancellation is preserved
• canonicalize Arcane server identity and scope encrypted credentials, legacy migration, persisted environment state, and process caches to that identity
• rotate client/session ownership during server changes, reject stale asynchronous results, and durably clear the saved server and credential binding before setup is shown
• refresh the pinned iOS 0.7 parity analysis and canonical Android task roadmap

Validation

• ./gradlew --no-daemon :app:testDebugUnitTest :app:assembleDebug
  • 120 tests passed with zero failures, errors, or skips
  • debug APK assembled successfully
• git diff --check
• physical-device checks:
  • password login succeeded
  • valid-session relaunch and process recreation restored without a login flash
  • logout/relaunch did not flash authenticated content
  • Change Server exposed blank setup with no prior credentials or content
  • immediate force-stop/relaunch after Change Server retained blank setup

OIDC is not configured on the test server. A live switch to a second server origin and manual equivalent-URL check were not performed; two-origin isolation and equivalent URL spellings are covered by the focused JVM test matrix.

[mkaltner]

@greptile