feat: scaffold YShell engineering baseline

[delichik]

Motivation

• Provide an engineering baseline that implements the design docs' Tauri + xterm.js stack and interaction surface so implementation work can proceed from a concrete scaffold.
• Establish typed frontend models, IPC boundaries and a visual shell (sessions sidebar, workspace, tabs, terminal pane, quick-connect, settings, status bar) to validate UI/IPC shape.
• Create a Rust/Tauri backend skeleton with clear module boundaries and a runtime registry to own PTY/SSH responsibilities for follow-up implementation.

Description

• Added frontend scaffold including package.json, vite.config.ts, tsconfig.json, index.html, src/main.tsx, application shell and components under src/ (App, Workspace, TerminalPane, QuickConnectPanel, SessionSidebar, SettingsPanel, StatusBar), typed models in src/bindings/types.ts, and IPC wrappers in src/bindings/ipc.ts with a browser fallback.
• Integrated xterm.js placeholder terminal rendering in TerminalPane and application styles in src/styles/global.css, and added a saveSession flow from quick connect to IPC.
• Added Tauri Rust backend skeleton under src-tauri/ including Cargo.toml, tauri.conf.json, src-tauri/src/lib.rs, src-tauri/src/main.rs, commands (sessions_list, sessions_save, settings_load, terminal_open_local, terminal_open_ssh, terminal_close), configuration models (config/mod.rs) and a terminal::RuntimeRegistry to track runtimes.
• Project housekeeping: added .gitignore and a small src/features/logs/README.md describing future logging UI responsibilities.

Testing

• Ran cargo fmt, which completed successfully.
• Ran npm install, which could not complete because the npm registry returned HTTP 403 and frontend dependencies (e.g., @tauri-apps/api) could not be downloaded.
• Ran npm run typecheck (tsc --noEmit), which failed because the frontend dependencies/types were not available due to the blocked npm install.
• Ran cargo check, which failed because crates.io index download was blocked (HTTP 403) and Rust dependencies could not be fetched.

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Codex Task