Gate Sparkle auto-install on Rsnap idle state

[yvette-carlisle]

Summary

• use Sparkle's immediate install handler for automatic updates instead of adding custom Rsnap prompts
• defer automatic install/relaunch while capture, quick screenshot, Settings, or permission recovery UI is active
• update the local Sparkle smoke wording and Settings spec for the idle-gated automatic flow

Verification

• cargo make fmt-swift
• cargo make check-swift
• cargo make test-macos-native-host
• cargo make test-macos-native-host-stage
• scripts/smoke/sparkle-update-local.sh --self-check
• git diff --check
• semantic drift audit: pass
• staged app Info.plist readback: SUAutomaticallyUpdate=true, SUScheduledCheckInterval=86400