refactor: remove obsolete helper modules

[Foxpace]

Removes two standalone helper modules whose responsibilities are now part of the main architecture.

What was added

• Removal of the CountDownDialog module and its UI resources and tests.
• Removal of the flipper storage compatibility module and Java file helpers.

How it works

Recording timing is rendered inside the Compose measurement flow, while document creation and writing use core NativeDocumentStorage and the Android document APIs. The application therefore no longer needs separate countdown UI or custom storage compatibility libraries.

Stack position

Layer 30 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Superseded by #46, which targets master and shows the final current tree.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 The architecture work continues as stacked PRs #47 through #54; consolidated PR #46 is superseded.