refactor(engine): theme is a UI preference; remove it from GameState

Previously, clicking the theme button updated localStorage and the DOM but
not state.theme, so the engine's `theme` meta-verb toggled from stale state.
Theme is now exclusively UI/storage concern. Old saves with the field still
load; the field is silently ignored.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/engine/save.test.ts

@@ -14,7 +14,6 @@ const baseState = (overrides: Partial<GameState> = {}): GameState => ({
   lastNoun: null,
   pendingDisambiguation: null,
   transcript: [],
-  theme: 'amber',
   endedWith: null,
   ...overrides,
 })