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.

dev.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+"""Dev wrapper for cargo tauri dev. Ctrl+C kills all child processes."""
+import subprocess, signal, os, sys, time
+
+# Ensure terminal delivers SIGINT on Ctrl+C (zsh disables this in some configs)
+os.system("stty isig 2>/dev/null")
+
+proc = subprocess.Popen(
+    ["cargo", "tauri", "dev"] + sys.argv[1:],
+    start_new_session=True,
+)
+
+def kill_all(sig, frame):
+    try:
+        os.killpg(proc.pid, signal.SIGKILL)
+    except ProcessLookupError:
+        pass
+    sys.exit(0)
+
+signal.signal(signal.SIGINT, kill_all)
+signal.signal(signal.SIGTERM, kill_all)
+
+while proc.poll() is None:
+    time.sleep(0.5)