fix(example): strip file:// prefix from DocumentPicker URI

Stage B device measurement surfaced a bug in the native module:
ExpoLitertLmModule.swift:96 calls `URL(fileURLWithPath: modelPath)`,
which interprets its arg as a literal POSIX path. When
DocumentPicker.getDocumentAsync({copyToCacheDirectory: false})
returns a file:// URI string, the constructed URL has a malformed
`.path` that FileManager.fileExists(atPath:) returns false for —
surfacing in JS as "LiteRT-LM model file not found".

This commit applies the workaround in the example so device
measurement can complete: strip `file://` from the picker URI before
setModelPath. Metro hot-reloads; no rebuild needed.

The proper fix in the native module — accept either a POSIX path or
a file:// URL — lands in 0.2.0-rc.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: helenkwok

example/App.tsx

@@ -82,7 +82,12 @@ export default function App() {
     });
     if (res.canceled) return;
     const uri = res.assets?.[0]?.uri;
-    if (uri) setModelPath(uri);
+    if (uri) {
+      // Native module's URL(fileURLWithPath:) treats its arg as POSIX path,
+      // so a file:// scheme prefix breaks fileExists. Strip it for Stage B.
+      // Phase 14-09 follow-up: native module should accept either form.
+      setModelPath(uri.startsWith('file://') ? uri.replace(/^file:\/\//, '') : uri);
+    }
   };
 
   const startPolling = () => {