unpin lex-client, drop 0.2.4 export-car workaround

The pnpm override pinned @atproto/lex-client to 0.2.4, predating both
lex-client 0.3.0 and sdk preview.2 - which assumes 0.3.0 semantics
(per-call service: null is a no-op on 0.2.4). 0.3.0 also returns
non-JSON encodings raw, so the ExportCarDialog fetchHandler
workaround for mis-parsed application/jsonl is unnecessary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pnpm-workspace.yaml

@@ -4,9 +4,6 @@ strictPeerDependencies: false # default: false
 trustPolicy: 'no-downgrade' # default: off
 trustPolicyIgnoreAfter: 10080 # 7 days, default: undefined
 overrides:
-  '@atproto/lex-schema': '0.2.2'
-  '@atproto/lex-data': '0.1.5'
-  '@atproto/lex-client': '0.2.4'
   '@react-native/babel-preset': '0.81.5'
   '@react-native/normalize-colors': '0.81.5'
   '@expo/image-utils': '0.8.12'