✨ overlay opacity: global setting + settings screen (Issue #16)

- Add UI_SETTINGS_KEY to config.ts for separate localStorage entry
- Add uiOpacity field (default 0.8, range 0.4–1.0, 10 % steps) to UIScene
- loadUISettings / saveUISettings persist opacity independently of game save
- Replace all hardcoded panel BG alphas with this.uiOpacity:
  build menu, villager panel, context menu, ESC menu, confirm dialog,
  nisse info panel
- Debug panel (F3) background synced via updateDebugPanelBackground()
- Replace Settings toast with real Settings overlay:
  title, opacity − / value / + buttons, Close button
- ESC key priority stack now includes settingsVisible
- repositionUI closes settings panel on window resize

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/config.ts

@@ -49,6 +49,9 @@ export const VILLAGER_NAMES = [
 export const SAVE_KEY = 'tg_save_v5'
 export const AUTOSAVE_INTERVAL = 30_000
 
+/** localStorage key for UI settings (opacity etc.) — separate from the game save. */
+export const UI_SETTINGS_KEY = 'tg_ui_settings'
+
 /** Milliseconds for one tree-seedling stage to advance (two stages = full tree). */
 export const TREE_SEEDLING_STAGE_MS = 60_000   // 1 min per stage → 2 min total