feat(gui): add Tauri desktop app

[pranavp311]

Summary

Add a Tauri-based desktop GUI for Codegraff, including project/session management, chat streaming, terminal panes, settings, and desktop bridge commands.

Context

This introduces a desktop app surface for the release branch while keeping the GUI crate isolated from the root workspace. The branch is rebased on release/0.2.15.

Changes

• Adds the React/Vite GUI under gui/ with project onboarding, sidebar navigation, chat surfaces, settings, and workspace panes.
• Adds the Tauri crate under gui/src-tauri with runtime/session state, command bridges, DTO contract generation, persistence, and desktop integrations.
• Adds generated frontend desktop contracts and test coverage for session state, chat work rows, activity results, and file diff rendering.
• Updates the macro path handling needed for generating desktop contracts from the nested GUI workspace.

Testing

Validated locally against release/0.2.15:

cd gui
npm run lint
npm test -- --run
npm run build

cd ..
cargo fmt --manifest-path gui/src-tauri/Cargo.toml -- --check
cargo check --manifest-path gui/src-tauri/Cargo.toml
cargo test --manifest-path gui/src-tauri/Cargo.toml
cargo clippy --manifest-path gui/src-tauri/Cargo.toml -- -D warnings

All checks passed.