refactor(desktop): reuse packages/server TLS assets in Tauri

Load server-managed TLS certificates (server-cert.pem, server-key.pem,
ca-cert.pem) from the server's TLS directory instead of generating a
separate proxy certificate in Tauri. Also trust the server CA in the
Windows trust store instead of a self-signed proxy cert.

This aligns with the reviewer feedback to avoid duplicating certificate
management across the codebase.

packages/tauri-app/src-tauri/Cargo.toml

@@ -14,9 +14,9 @@ serde_json = "1"
 serde_yaml = "0.9"
 axum = "0.7"
 axum-server = { version = "0.7", features = ["tls-rustls"] }
+base64 = "0.22"
 bytes = "1"
 futures-util = "0.3"
-rcgen = "0.13"
 rustls = { version = "0.23", features = ["ring"] }
 reqwest = { version = "0.12", default-features = false, features = ["http2", "charset", "json", "stream", "rustls-tls"] }
 rand = "0.8"