feat: support theme for components not using ThemableMixin

[web-padawan]

Problem

Fixes #393.

Switch and Breadcrumbs support the theme attribute in their web-component API but do not use ThemableMixin (planned for removal in Vaadin 26), so their element classes do not expose ThemePropertyMixinClass. The conditional type in createComponent gates the theme prop on that mixin:

type AllWebComponentProps<I, E> = I extends ThemePropertyMixinClass
  ? ThemedWebComponentProps<I, E>   // adds theme?: string
  : ComponentProps<I, E>;           // no theme

As a result, <Switch theme="…"> / <Breadcrumbs theme="slash"> produced Property 'theme' does not exist …, even though the attribute works at runtime.

Solution

• createComponent.ts — relax ThemedWebComponentProps to I extends HTMLElement; add ReactWebComponent / ThemedReactWebComponent types and a createThemedComponent() helper that always exposes theme at the type level. Runtime is identical to createComponent (unknown props already flow to the DOM as attributes via @lit/react), so this is a type-only fix.
• scripts/utils/settings.ts — new themedElements registry (Switch, Breadcrumbs) opting components into the themed variant.
• scripts/generator.ts — emits createThemedComponent + theme?: string for registry members; all other ~80 components are unchanged.

This decouples theme support from the mixin type and gives a clean migration path: as Vaadin 26 removes ThemableMixin, components move into themedElements instead of silently losing theme. The conditional-type path for existing mixin components is untouched.

Tests

• Extended test/ThemedReactWebComponent.spec.tsx with Switch (theme="small") and Breadcrumbs (theme="slash") cases.
• Added dev/pages/Switch.tsx and dev/pages/Breadcrumbs.tsx for manual testing.

Verification

• npm run validate:types → pass
• npm test → 17 files / 104 tests pass (new spec 3/3)
• Prettier clean

🤖 Generated with Claude Code