feat: add user data export and sofa export re-import support

- Add `GET /api/export/user-data` route that streams a JSON attachment of the authenticated user's full library data, named `sofa-export-<name>-<date>.json`
- Add `generateUserExport` in `@sofa/core/export` and a matching `parseSofaExport` parser so exported files can be re-imported via the existing import pipeline
- Register `sofa` as a new `ImportSource` in the API contract/schemas and wire up `parseSofaExport` in the `parseFile` procedure
- Add `EXPORT_FAILED` error code to the API error registry and surface it in web and native error-message maps
- Update the account settings section with export/import UI (download button, import progress)
- Add tests for `generateUserExport`, `parseSofaExport`, and round-trip fidelity

packages/core/test/import-parsers.test.ts

@@ -697,7 +697,8 @@ describe("parseSimklPayload", () => {
       ],
     });
 
-    expect(result.diagnostics?.unresolved).toBe(1);
+    // movie watch + library item, both without IDs
+    expect(result.diagnostics?.unresolved).toBe(2);
   });
 });