Fix workout recovery race conditions and add validation for corrupted state

[Copilot]

Addresses four critical issues in workout session recovery and auto-save identified in PR #76 review.

Changes

Data Loss Prevention in endWorkout

• Stop auto-save interval synchronously before any state mutation
• Clear localStorage only after successful DB write (try-catch protected)
• Preserves recovery state on DB errors

// Stop interval first to prevent race conditions
if (autoSaveTimerRef.current) {
  window.clearInterval(autoSaveTimerRef.current);
  autoSaveTimerRef.current = null;
}

try {
  await createWorkout(completedWorkout);
  clearAutoSavedWorkout();  // Only clear after success
} catch (error) {
  // Workout remains in localStorage for recovery
  throw error;
}

Auto-Save Closure Stale State Fix

• Use refs (workoutRef, isActiveRef) to track current state
• Auto-save callback reads from refs instead of closure-captured values
• Eliminates window where stale data could be saved after workout ends

Validation for Corrupted Recovery State

• Reject non-object JSON values (null, primitives, arrays)
• Auto-clear corrupted data from localStorage
• Type-safe casting only after validation

// Validate before accessing properties
if (!workout || typeof workout !== 'object' || Array.isArray(workout)) {
  localStorage.removeItem('activeWorkout');
  return null;
}

Test Coverage

• 14 new unit tests covering edge cases: malformed JSON, type mismatches, completed workouts, Date revival
• All localStorage error paths validated

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