Use queryProductDetailsAsync instead of querySkuDetailsAsync. Deadline is August 31, 2026

### Description
The current implementation uses the deprecated `querySkuDetailsAsync` method from the Google Play Billing Library. Google recommends migrating to `queryProductDetailsAsync`, which is required for newer versions of the API.

Billing Library version 7 is supported until **August 2026**, but **version 8 requires `queryProductDetailsAsync`** and no longer supports the old SKU-based API.

### Why this matters
- `querySkuDetailsAsync` is deprecated
- Billing Library 8 does not support it
- Ensures compatibility with upcoming Play Store requirements

### Proposed Solution
- Replace `querySkuDetailsAsync` with `queryProductDetailsAsync`
- Migrate from `SkuDetails` to `ProductDetails`
- Update related purchase flow logic

### Acceptance Criteria
- No usage of `querySkuDetailsAsync`
- All queries use `queryProductDetailsAsync`
- Purchase flows work as expected

### Alternatives

Newer subscription products don't appear by default when using the deprecated API. They must be explicitly marked as backward compatible in the Google Play Console to be returned via `querySkuDetailsAsync`. This will stop working on August 2026.