feat: add interactive update check on startup

When starting kctl-tui without a subcommand, check GitHub Releases
for a newer version and prompt the user to update.

- Silent skip for dev builds and network errors
- Prompt: 'Update now? [y/N]'
- On 'y': apply update, then start TUI
- On 'N'/Enter: start TUI immediately

go.mod

@@ -3,6 +3,7 @@ module github.com/skoelle/kctl-tui
 go 1.25.12
 
 require (
+	github.com/Masterminds/semver/v3 v3.5.0
 	github.com/charmbracelet/bubbles v0.21.1
 	github.com/charmbracelet/bubbletea v1.3.10
 	github.com/creativeprojects/go-selfupdate v1.6.0
@@ -12,7 +13,6 @@ require (
 require (
 	code.gitea.io/sdk/gitea v0.23.2 // indirect
 	github.com/42wim/httpsig v1.2.4 // indirect
-	github.com/Masterminds/semver/v3 v3.5.0 // indirect
 	github.com/atotto/clipboard v0.1.4 // indirect
 	github.com/aymanbagabas/go-osc52/v2 v2.0.1 // indirect
 	github.com/charmbracelet/colorprofile v0.4.1 // indirect