feat(tui): add theme system with 4 curated palettes

Flexoki Dark (default), Flexoki Light, Catppuccin Mocha, Nord.
Press T to cycle themes; selection persists in preferences.

All hardcoded colors replaced with theme-driven values.
Dedicated ZebraBg per theme for subtle row striping.

internal/tui/tab_users.go

@@ -94,7 +94,7 @@ func (m *Model) initUserHuhForm() tea.Cmd {
 					huh.NewOption("Admin", "admin"),
 				).Value(&m.userFormData.Role),
 		).Title("SSH Access"),
-	).WithTheme(huh.ThemeDracula())
+	).WithTheme(m.theme.HuhTheme())
 
 	return m.huhForm.Init()
 }