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/types.ts

@@ -30,7 +30,6 @@ export type ParsedCommand =
   | { kind: 'unknown'; raw: string; reason: 'unknown-verb' | 'unknown-noun' | 'malformed' }
 
 export type ResolveLevel = 'steady' | 'shaken' | 'reeling' | 'returning'
-export type Theme = 'amber' | 'ansi'
 
 export interface ItemInstance {
   id: ItemId
@@ -68,7 +67,6 @@ export interface GameState {
   pendingDisambiguation: PendingDisambiguation | null
   /** Capped at 200 entries; older entries are dropped on append. */
   transcript: TranscriptLine[]
-  theme: Theme
   /** Set true when the player has reached an ending. UI shows ending screen. */
   endedWith: 'true' | 'wrong' | 'bad' | null
 }