Add CI: fmt, clippy, test, frontend build

GitHub Actions on push/PR (macOS runner): checks out the vendored SDK
submodule (full history so its pinned fork-branch commit is reachable),
builds the frontend (tauri-build needs ui/dist), then runs
`cargo fmt --check`, clippy (advisory for now), and
`cargo test --workspace`. Caches cargo to keep runs reasonable.

Also point .gitmodules at `tutabridge-integration` (the branch the
submodule commit actually lives on).

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "tuta-repo"]
 	path = tuta-repo
 	url = https://github.com/spartanz51/tutanota.git
-	branch = feat/rust-sdk-blob-read
+	branch = tutabridge-integration