feat: scaffold Tauri desktop GUI (Svelte + TypeScript)

[pszymkowiak]

Summary

• Adds `crates/enigma-gui/` — a Tauri 2 desktop application wrapping the existing CLI flows (init, backup, restore, list, status, config view).
• Frontend: Svelte 4 + TypeScript + Vite. Backend: Rust, calls `enigma-core` / `enigma-storage` / `enigma-keys` directly with no IPC ceremony.
• New crate added to workspace `members`; excluded from `default-members` so plain `cargo build` at the root does not require the Tauri toolchain (webkit2gtk on Linux, etc.).

What's in

Layer	Files
Rust backend	`src/main.rs`, `src/commands.rs` (7 `#[tauri::command]` handlers), `src/state.rs`, `tauri.conf.json`, `build.rs`, `capabilities/default.json`
Svelte frontend	`ui/` — `App.svelte` + 7 route components (`Welcome`, `Init`, `Backup`, `Restore`, `BackupList`, `Status`, `ConfigView`)
TS plumbing	`ui/src/lib/invoke.ts` (typed Tauri wrappers), `ui/src/lib/types.ts` (DTOs mirroring Rust structs)
Workspace	`Cargo.toml` adds `crates/enigma-gui` and a `default-members` list

Test plan

• `cd crates/enigma-gui/ui && npm install && npm run build` succeeds
• `cargo check -p enigma-gui` succeeds (not validated locally — dev env missing build-essential)
• `cargo tauri dev` launches the window (needs webkit2gtk on Linux)
• Click through: Init flow → enter passphrase → Backup tab → pick a folder → run → check BackupList shows the record

Out of scope (follow-ups)

• System tray + scheduled backups + notifications
• Integration of the new OneDrive / Google Drive backends (parallel PR)
• Code signing for macOS / Windows
• E2E tests (Playwright / WebdriverIO)
• Localization (i18n)

Known limitations

• Not validated locally (no build-essential on dev machine).
• The `init_providers` match in `src/commands.rs` currently lists `Azure` and `Gcs` as `bail!` cases; the parallel PR A adds `Onedrive` and `Gdrive` variants to `ProviderType` which will force this match to be updated when PR A merges first.

🤖 Generated with Claude Code