fix(privacy): decide and fix iOS UIFileSharingEnabled / LSSupportsOpeningDocumentsInPlace

## Problem
(verified) `LSSupportsOpeningDocumentsInPlace` (`ios/Runner/Info.plist:66-67`) and `UIFileSharingEnabled` (`Info.plist:127-128`) are both true — anyone with the unlocked phone (or a synced computer) can browse exported app documents, which include cached study data files.

## Proposed fix
Confirm whether this was intentional (e.g., a debugging convenience); if not, set both false. If data export *is* a feature, scope it to an explicit share action instead.

## Acceptance criteria
- [ ] Decision recorded
- [ ] Flags match the decision; cached JSON not browsable via Finder when disabled

**Effort:** S · **Priority:** P2 · **Change risk:** Low