Add auto-update support (tauri-plugin-updater) and Gitea release pipeline

Ed25519-signed updates, checked via a fixed-tag latest.json manifest on
Gitea Releases; scripts/release.sh automates build+sign+publish. Adds
a manual "Nach Updates suchen" trigger plus a silent startup check.

src-tauri/Cargo.toml

@@ -25,6 +25,8 @@ tauri = { version = "2.11.3", features = [] }
 tauri-plugin-log = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-notification = "2"
+tauri-plugin-updater = "2"
+tauri-plugin-process = "2"
 tokio = { version = "1", features = ["full"] }
 imap = "3.0.0-alpha.15"
 native-tls = "0.2"