fix: share via own FileProvider to prevent ResultActivity crash

Sharing ML Kit/gallery content URIs directly with a grant flag threw
SecurityException (URIs we do not own cannot be re-granted), crashing
ResultActivity back to the home screen. Copy pages/PDF into our cache
and share through our FileProvider URI. Wrap the share call so any
failure shows a toast instead of crashing.

Constraint: share never crashes the activity; reading source URIs already works (save/OCR paths).
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: user re-tests share on device with refreshed APK.
Tested: build + 6 unit tests pass; device share pending user re-test.
Not-tested: on-device share sheet (no device on server).
Related: ClearCam MVP, planned FileProvider fallback

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