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.

ui/tsconfig.json

@@ -0,0 +1,7 @@
+{
+  "files": [],
+  "references": [
+    { "path": "./tsconfig.app.json" },
+    { "path": "./tsconfig.node.json" }
+  ]
+}