cleanup: `updateDerivedScreens` recreates Settings and Help screens on every call

[gitcoder89431]

Problem

updateDerivedScreens in internal/app/update.go unconditionally calls screens.NewSettings(...) every time it's invoked (on every theme change, sidebar toggle, resize, and route change):

func (m *Model) updateDerivedScreens() {
    ...
    m.screens["settings"] = screens.NewSettings(screens.SettingsState{...}, m.theme) // always new
    if s, ok := m.screens["help"].(screens.Help); ok {
        m.screens["help"] = s.WithTheme(m.theme) // correct pattern
    } else {
        m.screens["help"] = screens.NewHelp(m.keys.FullHelp(), m.theme) // only on first call
    }
}

Settings is always reconstructed (losing any in-progress state), while Help and Activity correctly use a WithTheme pattern. Settings should follow the same WithTheme pattern, or at minimum use the type-assert-then-update approach.

Fix

Give Settings a WithTheme and WithState method (or a Update(SettingsState) method) so it can be updated in-place rather than reconstructed from scratch on every derived screen update.

[gitcoder89431]

Fixed in f9cf96a