Prepare 0.2.1 release

[jsgrrchg]

Summary

• add user-facing release notes for version 0.2.1
• align Electron and Rust workspace versions with the release
• refresh native backend capability fixtures and contract expectations

Validation

• pnpm install --frozen-lockfile
• pnpm run typecheck
• pnpm run lint (passes with two existing React warnings)
• pnpm run test
• pnpm run build:ci
• cargo check --workspace
• cargo test --workspace
• release notes extraction and release body generation for v0.2.1