catch SecurityException on media URI access in the share flow

[munzzyy]

Fixes #137.

The crash report's java.lang.SecurityException: org.witness.proofmode has no access to content://media/external/video/media/1000011572 comes from ContentResolver.openInputStream(). That method documents SecurityException separately from FileNotFoundException, thrown when the caller lacks a valid grant for the URI. A content://media/... URI like this one is exactly what you get from Intent.EXTRA_STREAM on an incoming ACTION_SEND, and its read grant is temporary, tied to the sending app's intent rather than persisted. If that grant is gone by the time ProofMode tries to read it, openInputStream throws SecurityException.

ShareProofActivity only catches FileNotFoundException around these openInputStream calls, in checkProof()/proofExists() (runs automatically from onResume() the moment the share screen opens), generateProof(), and generateMultiProof(). Since SecurityException is a RuntimeException, not an IOException, none of those catch blocks touch it, so it escapes the coroutine and crashes the app. That matches a crash that fires right on opening the share screen, before the user does anything.

Fix: catch SecurityException alongside the existing FileNotFoundException in those three spots, with the same fallback that's already there for a missing file. Fail this one media item gracefully instead of taking the app down.

What I checked:

• Traced the R8-mapped stack trace by hand against current main. The innermost real frame lines up with proofExists()'s openInputStream call inside checkProof()'s coroutine, matching the IO-dispatcher frames beneath it. checkProof() runs unconditionally in onResume() for both ACTION_SEND and ACTION_SEND_MULTIPLE.
• Confirmed openInputStream's documented SecurityException behavior.
• Compiled the exact catch-clause pattern standalone with kotlinc to confirm it's valid Kotlin. Order doesn't matter here since FileNotFoundException and SecurityException aren't related types.
• Could not run the full :app Gradle build here. plugin-location-protocol depends on the generated :flutter module, which needs the Flutter SDK to scaffold flutter-location-protocol/.android, not installed in this environment and unrelated to this change. If you can run ./gradlew :app:compileDebugKotlin locally that would be good extra confirmation.