refactor(tui): restructure site form to 2 type-aware pages

Replace 4-page paginated form (17 fields for HTTP) with a 2-page
type-aware layout. Page 1 shows core fields + type-specific target
(URL for HTTP, Hostname for ping, etc). Page 2 shows configuration
with pre-filled defaults. Group type gets 1 page.

Form rebuilds dynamically when monitor type changes, preserving
all entered values via pointer-bound siteFormData. Focus returns
to the Type select after rebuild so users can continue forward.
WithWidth set explicitly on rebuild to prevent placeholder truncation.

internal/tui/tui.go

@@ -115,6 +115,7 @@ type Model struct {
 
 	huhForm       *huh.Form
 	siteFormData  *siteFormData
+	lastSiteType  string
 	alertFormData *alertFormData
 	userFormData  *userFormData
 	maintFormData *maintFormData