Refactor Sleep stage breakdown UI

[tigercraft4]

What this PR does

Moves the Sleep screen stage breakdown and hypnogram rendering helpers into SleepStageBreakdownUi.kt. This keeps the stage UI cluster beside the existing Sleep logic helper files and reduces SleepScreen.kt without changing behavior, calculations, persistence, or BLE paths.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• CI / tooling

How it was tested

• git diff --check
• python3 Tools/i18n_audit.py --ci origin/main
• JAVA_HOME=/opt/homebrew/Cellar/openjdk@21/21.0.11/libexec/openjdk.jdk/Contents/Home ANDROID_HOME=$HOME/Library/Android/sdk ./gradlew --dependency-verification=off :app:compileFullDebugKotlin from android/

Android unit tests were not run for this UI-only extraction; the current test task is already blocked by the existing DeviceRegistryDao.setModel(id, model) fake DAO mismatch tracked separately.

Checklist

• Swift package tests pass for any package I touched (swift test in Packages/<name>)
• Android unit tests pass if I touched android/ (./gradlew testFullDebugUnitTest)
• No new build warnings introduced
• UI changes use only StrandDesign tokens — no hardcoded colors, fonts, or spacing
• No hardcoded hex frame bytes; protocol facts live in the schema / decoders
• Follows the conventions in docs/CONTRIBUTING.md
• I did not commit generated output (Strand.xcodeproj/) or any secrets/keystores

Related issues

Refs #665