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/test/bvt/playwright.config.ts

@@ -0,0 +1,31 @@
+import { defineConfig } from '@playwright/test';
+import { fileURLToPath } from 'node:url';
+import * as path from 'node:path';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+export default defineConfig({
+  testDir: '.',
+  fullyParallel: true,
+  forbidOnly: false,
+  retries: 1,
+  workers: 1,
+  use: {
+    trace: 'on-first-retry',
+    screenshot: 'only-on-failure',
+  },
+  webServer: {
+    command: 'npx sirv build --port 18080',
+    port: 18080,
+    reuseExistingServer: true,
+    cwd: path.resolve(__dirname, '../../'),
+    stdout: 'ignore',
+    stderr: 'pipe',
+  },
+  projects: [
+    {
+      name: 'chromium',
+      use: { browserName: 'chromium' },
+    },
+  ],
+});