chore: migrate from react-native-fs to expo-file-system

[janicduplessis]

Fixes APP-####

What changed (plus any additional context for devs)

Removes react-native-fs from the app and routes every caller through expo-file-system, which is already pulled in transitively via the expo SDK and is now promoted to a direct dependency. The motivation was twofold: react-native-fs is unmaintained and needed a local patch to survive RN 0.81 (PromiseImpl.reject now rejects null code), and the API surface we use is small and maps cleanly to expo-file-system.

Changes by call site:

• src/components/DappBrowser/screenshots.ts — copyFile/unlink/readDir → copyAsync/deleteAsync({ idempotent: true })/readDirectoryAsync. The orphan-prune path was reshaped slightly: readDirectoryAsync returns plain filename strings, where RNFS returned ReadDirItem objects with a path field, so the file URIs are now built explicitly from documentDirectory + name.
• src/handlers/cloudBackup.ts — writeFile/unlink → writeAsStringAsync/deleteAsync. RNCloudFs's sourcePath.path still expects a POSIX path, so the file:// URI is stripped before the cloud copy.
• src/model/migrations.ts (migration v16) — unlink(path.join(...)) → deleteAsync(URI, { idempotent: true }). cacheDirectory already ends with a trailing slash, so path is no longer needed.
• src/handlers/pinata.ts — RNFS.uploadFiles({...}).promise → FileSystem.uploadAsync(url, fileUri, { uploadType: MULTIPART, ... }). expo-file-system derives the multipart filename from the URI's basename and has no override; to preserve the caller-supplied filename, it's passed through pinataMetadata.name in the form parameters, which Pinata uses for the pin name.
• src/screens/Diagnostics/helpers/createAndShareStateDumpFile.ts — same documentDirectory/writeAsStringAsync/deleteAsync swap. The Android-only RNFS.DownloadDirectoryPath write was redundant with the share sheet that follows and is dropped — expo-file-system has no equivalent for the public Downloads folder, but the cross-platform share sheet ("Save to Files", Drive, AirDrop, etc.) already covers the developer-export use case on both platforms.

react-native-fs is removed from package.json; base-64 (its sub-dep) drops out of the lockfile too.

Screen recordings / screenshots

n/a — equivalent behaviour, no UI surface change.

What to test

• DappBrowser screenshots: open and close several browser tabs over a session; verify tab thumbnails appear correctly when switching between tabs and that orphan cleanup still runs (no leftover screenshot files in the document directory after a tab is closed).
• Cloud backup: trigger an iCloud / Google Drive backup of the wallet and verify the file lands in the cloud and the local document copy is deleted afterwards.
• Migration v16: install an older build that still writes rainbow-token-list.json / rainbow-token-list-etag.json into the cache dir, then upgrade to this build; verify those files are gone after first launch (or simply that the migration runs without error in a fresh install).
• Pinata avatar upload: through the ENS profile flow, pick an image and upload it as the avatar; verify the upload succeeds, the IPFS URL resolves to the image, and (if you have access to the Pinata dashboard) the pin's name matches the original filename.
• Diagnostics state dump: open the Diagnostics screen and trigger "Share state dump"; verify the share sheet opens with the JSON file and that triggering it twice in a row works (no leftover-file errors).