Use nil for object pointers and HKSampleType for generic sample iteration

[Copilot]

Addresses code review feedback on PR #15 regarding Objective-C pointer conventions and type safety in HealthKit data fetching.

Changes:

• Changed fetchEndDate = NULL to fetchEndDate = nil in initialization
• Changed loop iterator from HKQuantityType * to HKSampleType * in readDataWithDataTypes:

Rationale:

• nil is the standard for Objective-C object pointers vs NULL for C pointers
• HKSampleType * correctly handles all HealthKit sample types (quantity, category, correlation, workout) returned by dataTypesToRead, not just quantities

- (void) readDataWithDataTypes: (NSSet *) types {
-    for (HKQuantityType * set in types) {
+    for (HKSampleType * set in types) {
        // Process any sample type, not just quantities
    }
}

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[tetujin]

LGTM