fix(gate-b): Svelte 5 event directives + label/input association for Select Photos

- Add id='photo-file-input' to file input and for='photo-file-input' to label
- Convert on:change → onchange, on:click → onclick, on:input → oninput
  (Svelte 5 requires native DOM event properties, not on: directives)
- Add Playwright BVT suite confirming file selection → mask step advance
- Test image: 100x60 PNG with green border (hue=66°) and white interior

frontend/package.json

@@ -11,10 +11,12 @@
     "test:watch": "vitest"
   },
   "devDependencies": {
+    "@playwright/test": "^1.63.0",
     "@sveltejs/adapter-static": "^3.0.6",
     "@sveltejs/kit": "^2.16.0",
     "@sveltejs/vite-plugin-svelte": "^5.0.3",
     "pngjs": "^7.0.0",
+    "sirv-cli": "^3.0.1",
     "svelte": "^5.0.0",
     "svelte-check": "^4.1.1",
     "typescript": "^5.6.3",