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/engine.test.ts

@@ -1,5 +1,9 @@
 import { describe, it, expect, afterEach } from 'vitest';
-import { createState, tick, buyUpgrade, type State } from './engine.ts';
+import { createState as createIntroState, startGame, tick, buyUpgrade, type State } from './engine.ts';
+
+function createState(): State {
+  return startGame(createIntroState());
+}
 import { UPGRADES } from './upgrades.ts';
 
 // Deterministic PRNG so simulation tests aren't flaky, but still exercise