Restructure into Cargo workspace with Tauri desktop GUI

Split the bridge into a tutabridge-core crate, a Tauri v2 desktop app
(src-tauri) and a React/TS UI (ui), keeping the CLI entrypoint at the
workspace root.

Add encrypted local storage (SQLCipher metadata index + encrypted .eml
files) so mail persists across launches and only the delta is fetched.

Wire the bridge to the Tuta Rust SDK via the tuta-repo submodule
(batch loading, MailDetailsBlob reading, interactive 2FA login).

Implement SMTP sending: build the draft and send it through Tuta's
DraftService/SendDraftService, mirroring the web client (body in
compressedBodyText, non-empty sender/recipient names, populated
SendDraftParameters). Add unit tests for the draft/send payload building.

src-tauri/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "tutabridge-gui"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.84.0"
+
+[build-dependencies]
+tauri-build = { version = "2", features = [] }
+
+[dependencies]
+tutabridge-core = { path = "../crates/bridge" }
+tauri = { version = "2", features = [] }
+tauri-plugin-shell = "2"
+tokio = { version = "1.43", features = ["full"] }
+tokio-rustls = { version = "0.26", features = ["ring"] }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+log = "0.4"
+env_logger = "0.11"