Restructure into Cargo workspace with Tauri desktop GUI (#1)

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/tauri.conf.json

@@ -0,0 +1,35 @@
+{
+  "$schema": "https://raw.githubusercontent.com/niceda/tauri/dev/crates/tauri-config-schema/schema.json",
+  "productName": "TutaBridge",
+  "version": "0.1.0",
+  "identifier": "com.tutabridge.app",
+  "build": {
+    "frontendDist": "../ui/dist",
+    "devUrl": "http://localhost:1420",
+    "beforeDevCommand": "cd ../ui && npm run dev",
+    "beforeBuildCommand": "cd ../ui && npm run build"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "TutaBridge",
+        "width": 700,
+        "height": 520,
+        "resizable": true,
+        "center": true
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png"
+    ]
+  }
+}