feat: add auto-update checker

Implements issue #17 - the app now checks for updates on startup and shows
a notification when a newer version is available. Users can disable this
in settings. Uses Gitea releases API with semver comparison.

Closes #17

src-tauri/Cargo.toml

@@ -25,7 +25,9 @@ uuid = { version = "1", features = ["v4"] }
 tauri-plugin-store = "2.4.2"
 tauri-plugin-notification = "2"
 tauri-plugin-os = "2"
+tauri-plugin-http = "2"
 tempfile = "3"
+semver = "1"
 chrono = { version = "0.4.43", features = ["serde"] }
 
 [target.'cfg(windows)'.dependencies]