Add units preference (kg/lbs toggle)

[Jds-23]

Summary

Add a user preference for weight units (kg vs lbs) with a toggle.

Tasks

• Add units preference state (kg or lbs)
• Store preference in AsyncStorage (or context if auth-scoped later)
• Add toggle UI in settings or profile area
• Update numpad modal to show selected unit label
• Update weight display in log session screen
• Update weight display in history/session detail
• Update weight display in progress charts
• Add unit conversion utility (kg ↔ lbs)

Acceptance Criteria

• User can toggle between kg and lbs
• Preference persists across app restarts
• All weight displays reflect the chosen unit
• Numpad shows correct unit label
• Conversion is accurate (1 kg = 2.20462 lbs)

Priority

P2 — Enhancement

Phase

Phase 10: Quality & Polish