ci: establish development tooling and CI baseline (issue #17)

[thundermiracle]

Summary

• extend package scripts with reproducible quality-gate entry points: ci:web, ci:rust, and ci
• add Rust quality scripts (fmt:rust, test:rust) and keep pnpm-first command flow
• pin Rust toolchain expectations in rust-toolchain.toml (stable + rustfmt/clippy)
• upgrade CI workflow to two jobs:
  • web-quality: lint + test + build
  • rust-quality: fmt + clippy + test (with Tauri Linux deps)
• document local quality-gate commands in README
• add contract tests for CI baseline and toolchain pinning

Validation

• pnpm run lint
• pnpm test
• cargo test --manifest-path src-tauri/Cargo.toml
• pnpm outdated

Closes #17