Add intro screen, win/loss art, and a way to abandon the ritual

- Adds a full-screen intro/title screen (new 'intro' game phase) with
  a moody Malmö/Turning Torso background and a "Börja ritualen"
  button; the game no longer starts ticking until the player begins.
- Win and loss overlays now show matching generated artwork behind
  the existing text, instead of a flat scrim.
- New "Avbryt" header button lets the player abandon a run mid-game
  (with a confirm prompt) via a new 'abandoned' phase, reusing the
  existing overlay/restart flow.
- Source images converted from PNG (~2.1MB each) to WebP (~120KB
  each) and placed under public/img/ so Vite serves them as-is.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main.ts

@@ -1,5 +1,5 @@
 import './style.css';
-import { createState, tick, buyUpgrade, visitLocation, resolveEventChoice, saveState, loadState, clearSave } from './engine.ts';
+import { createState, tick, buyUpgrade, visitLocation, resolveEventChoice, startGame, abandonGame, saveState, loadState, clearSave } from './engine.ts';
 import { initUI, renderState } from './ui.ts';
 import { initMap, updateMap } from './map.ts';
 
@@ -23,7 +23,9 @@ function start(): void {
       document.getElementById('overlay')!.classList.remove('visible');
       renderState(state);
     },
-    (index) => { state = resolveEventChoice(state, index); renderState(state); saveState(state); }
+    (index) => { state = resolveEventChoice(state, index); renderState(state); saveState(state); },
+    () => { state = startGame(state); renderState(state); saveState(state); },
+    () => { state = abandonGame(state); renderState(state); saveState(state); }
   );
 
   initMap('map', (locationId) => {