fix(export): share the fallback export without loading it into memory

[0xbrayo]

Part of #333.

#304 made large exports stream to a cache file so they are never held in memory as a String. The share-sheet fallback (used when CreateDocument can't be launched) still did file.writeText(pending.readContent()), which reads the entire export into a String and writes it back out, on the UI thread. A large bucket export on that path could OOM or ANR.

Change

• The fallback now moves the cached export into cacheDir/shared-exports/<uuid>/<filename> (moveExportForSharing) and shares it from there through FileProvider. A new cache-path entry covers that directory.
  • A rename within cacheDir is instant, so the share stays synchronous on the UI thread. There's no background work to race a second export or to be lost if the activity is recreated.
  • Each share gets its own directory, so exports with the same filename never overwrite each other.
  • Shared copies older than a day are removed on the next share.
• PendingExport.readContent() had no remaining production caller, so it is removed. The one test that used it now reads cacheFile directly.

Testing

• New WebUIFragmentTest cases: two exports with the same filename are shared as separate files with their own contents, and old shared exports are pruned.
• ./gradlew :mobile:testStandardDebugUnitTest passes.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable new issue remains.

Summary

The fallback now moves each cached export into its own sharing directory instead of loading the export into memory.

• Adds a matching FileProvider cache path.
• Removes shared exports older than a day on the next share.
• Adds tests for same-name exports and cleanup.
• Both earlier findings are addressed: exports no longer share a destination file, and no background copy can outlive the fragment.

_{Reviews (2) · Last reviewed commit: "fix(export): share the fallback export b..." · Reviewed by Greptile}

[0xbrayo]

🤖 Claude, on behalf of @0xbrayo

@greptile review