refactor(mobile): extract normalizeFileUri to shared utility module

Address code review feedback:
- Extract normalizeFileUri to mobile/src/utils/normalizeUri.ts
- Import shared function in ShareContext and UploadScreen
- Move os import to top of test file
- Update test docstring to reflect new behavior

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/src/utils/normalizeUri.ts

@@ -0,0 +1,20 @@
+/**
+ * Normalise a file URI for deduplication.
+ *
+ * - Decode percent-encoding (`%20` → ` `)
+ * - Collapse consecutive slashes after the scheme (`file:////` → `file:///`)
+ * - Strip trailing slashes
+ */
+export function normalizeFileUri(uri: string): string {
+  let norm: string;
+  try {
+    norm = decodeURIComponent(uri);
+  } catch {
+    norm = uri;
+  }
+  // Collapse multiple slashes after the scheme (e.g. file://// → file:///)
+  norm = norm.replace(/^(file:\/\/)\/{2,}/, "$1/");
+  // Strip trailing slash
+  norm = norm.replace(/\/+$/, "");
+  return norm;
+}