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/capabilities/default.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://raw.githubusercontent.com/niceda/tauri/dev/crates/tauri-utils/schema.json",
+  "identifier": "default",
+  "description": "Default capabilities for the main window",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "shell:allow-open",
+    "core:event:default",
+    "core:event:allow-listen",
+    "core:event:allow-emit"
+  ]
+}