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/update.go

@@ -128,6 +128,13 @@ func (m *Model) handleFormMsg(msg tea.Msg) (tea.Model, tea.Cmd) {
 		if f, ok := form.(*huh.Form); ok {
 			m.huhForm = f
 		}
+		if m.state == stateFormSite && m.siteFormData != nil &&
+			m.siteFormData.SiteType != m.lastSiteType {
+			rebuildCmd := m.rebuildSiteForm()
+			// Advance to Type select — user just changed it.
+			skipName := m.huhForm.NextField()
+			return m, tea.Batch(rebuildCmd, skipName)
+		}
 		if m.huhForm.State == huh.StateCompleted {
 			// The store write runs in the returned Cmd; its writeDoneMsg
 			// triggers the tab-data reload once the row actually exists.