fix: handle unicode redirect payloads in dapp transport

[Ysh204]

Summary

• Encode redirect request payloads as UTF-8 before base64 encoding
• Decode redirect response payloads as UTF-8 after base64 decoding
• Add regression coverage for Unicode redirect request and response payloads

Testing

• corepack pnpm --filter @0xsequence/dapp-client exec vitest run
• corepack pnpm --filter @0xsequence/dapp-client lint
• corepack pnpm --filter @0xsequence/dapp-client typecheck
• corepack pnpm --filter @0xsequence/dapp-client build

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[Ysh204]

Good catch. Updated the decoder to use fatal UTF-8 decoding and fall back to the legacy atob string when the payload is not valid UTF-8. I also added regression coverage for a legacy Latin-1 redirect response payload.