Migrate Cedar to native GPUI

[oshtz]

Summary

• replace the React/Tauri desktop shell with a native Rust + GPUI interface
• preserve Cedar's Cloudflare backend, local SQLite data, and keychain contracts
• add deterministic visual QA, native packaging, signing, checksums, and Windows/macOS release automation
• remove the Node, Vite, React, Tauri, WebView, and Enigma release paths

Validation

• cargo fmt --check
• cargo test --locked (26 passed)
• cargo clippy --locked --all-targets --all-features -- -D warnings
• packaged Windows GPUI startup smoke
• Windows archive and SHA-256 verification
• actionlint for CI and release workflows