refactor(app): remove legacy phone activities

[Foxpace]

Replaces the legacy phone activity flow with a single Compose application host.

What was added

• Removal of the old About and Measurement activities.
• Simplification of MainActivity around the Compose root application.

How it works

MainActivity now provides the Android lifecycle boundary and renders SensorBoxApp. Navigation, setup, active recording, about content, and back behavior are driven by the main contract and Compose navigation state rather than separate activities.

Stack position

Layer 22 of 31 in the SensorBox refactor stack. Review this PR against its configured base to see only this layer.

Validation

• Full stack: ./gradlew test — BUILD SUCCESSFUL (160 tasks)

[Foxpace]

Superseded by #46, which targets master and shows the final current tree. Closing this stacked layer so review stays on the consolidated diff; this PR remains available as history.

[Foxpace]

Stack restored at the author's request. This PR is again part of the original 31-layer refactor chain. The architecture work continues as stacked PRs #47 through #54; consolidated PR #46 is superseded.