Implement "Share as Image" for results

Users should be able to share their dyno graph as a clean, professional-looking image. Instead of a manual screenshot, we want a dedicated "Share" button that captures only the result card and the chart, then opens the native system share sheet.

**Requirements**

- Integrate react-native-view-shot for the capture.
- Use expo-sharing (or react-native-share) for the system dialog.
- Ensure the exported image has a clean background (even in Dark Mode).

Why this matters: this is a high-impact feature! Sharing dyno results is the primary way users interact with the community. A clean, branded image export will not only improve user experience but also act as organic growth for the project when shared on social media or automotive forums.