feat: add Health Connect session export

[0then0]

• Adds an Android-only Expo native module using Health Connect WRITE_MINDFULNESS.
• Adds a Settings toggle that requests permission before enabling export.
• Writes completed breathing exercises as mindfulness sessions without reading health data.
• Splits sessions around app background periods so inactive time is excluded.
• Handles unavailable providers, outdated Health Connect, revoked permission, invalid intervals, and native write failures.
• Persists the toggle safely with validation for malformed stored settings.
• Adds unit and component tests for permissions, errors, persisted state, background/resume segments, and clock changes.
• Raises Android minSdkVersion to 26, required by the Health Connect SDK.
• Ignores local native-module Gradle build output.

Checks before merge/release

• Confirm that storing a backgrounded exercise as multiple mindfulness records is acceptable. Background time is intentionally excluded.
• Verify the flow on a physical Android device:
  • Android 9–13 with the Health Connect app installed;
  • Android 14+ with built-in Health Connect;
  • permission grant, permission revocation, provider update requirement, and successful record creation.
• Review the Android compatibility change: Health Connect requires minSdkVersion 26, so Android 7/API 24–25 will no longer be supported.
• Before a Google Play release, declare WRITE_MINDFULNESS in Play Console and ensure its Health Connect privacy policy matches the text shown by the in-app rationale activity.
• Confirm whether partial export is acceptable when one segment saves and another fails. The app reports the failure but does not retry automatically.

Closes #130