Release v0.2.0 with Vocabulary Study

[Ldsystem]

Summary

• ship the accepted Vocabulary Study implementation accumulated since v0.1.1
• document personal memory, Simplified Chinese textbooks, related-word discovery, pronunciation, POS metadata, and bidirectional practice
• add three current screenshots captured from the installed macOS app
• synchronize package, Cargo, lockfile, and Tauri versions at 0.2.0

Validation

• pnpm check — 119 frontend tests passed; typecheck and production renderer build passed
• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path src-tauri/Cargo.toml — 129 passed, 7 documented manual fixtures ignored
• pnpm tauri build --target universal-apple-darwin
• universal app verified as x86_64 + arm64 with a valid ad-hoc signature and version 0.2.0
• README local links and screenshot dimensions verified; release version parity verified across all four metadata sources

Release

After required checks and review pass, this PR is intended to become GitHub release v0.2.0.