Add personalized HR zone thresholds (#138)

[kavemang]

Closes #138.

What changed

• adds an opt-in Custom HR zones editor to Profile settings on Apple and Android
• lets users set the inclusive BPM start for Zones 1–5, with neighbour-aware controls that keep all boundaries strictly ordered
• keeps the existing percentage-of-max model as the default and restores it immediately when customization is disabled
• routes live HR, live workouts, workout time-in-zone splits, and haptic zone coaching through the same effective zone set
• includes the personalized boundaries in cross-platform .noopbak settings backups
• adds German, Spanish, and French translations for the new UI

Why

The existing max-HR override helps users whose measured maximum differs from the age estimate, but it cannot represent lab- or threshold-derived training zones such as LT1/LT2. Personalized BPM boundaries provide that control without changing Effort/strain calculations or any stored workout data.

Safety and compatibility

• no behavior changes unless Custom HR zones is enabled
• invalid or malformed restored boundaries fall back to the conventional model
• no BLE, database migration, or medical classification changes

Validation

• ./gradlew testFullDebugUnitTest — passes
• python3 Tools/i18n_audit.py --ci upstream/main — passes
• git diff --check — passes
• PR Swift Packages CI — passes, including StrandAnalytics and WhoopStore
• on-demand App build — passes for universal macOS and iOS Simulator