refactor: remove tauri runtime and harden linux compatibility

[pallyoung]

Summary

• refactor server runtime from Tauri host to pure Rust Tokio/Axum server
• remove frontend and release pipeline Tauri fallback/dependencies
• harden Linux runtime compatibility by pinning CI/Release to Ubuntu 22.04
• switch Linux release builds to musl static target in CI/Release
• update packaging helpers so release artifacts resolve target-specific server binaries

Validation

• pnpm build:server
• pnpm test:cli
• pnpm test:e2e:release
• pnpm release:check

Notes

• local default build path remains unchanged for development
• Linux CI/Release now builds x86_64-unknown-linux-musl artifacts on Ubuntu 22.04