Connect to server over websocket for live cloud sync

Desktop app now holds a persistent websocket to the server and syncs
on push notifications instead of a fixed timer, falling back to the
old polling interval only when the socket is down. Cloud folder/file
listings are cached locally so the browser stays usable offline.

Claude-Session: https://claude.ai/code/session_01PoLmSR1pFVyHf8i5b1rNWk

src-tauri/Cargo.toml

@@ -42,4 +42,5 @@ base64 = "0.22"
 diffy = "0.4"
 walkdir = "2"
 ureq = { version = "2.12", features = ["json"] }
+tungstenite = { version = "0.24", features = ["native-tls"] }