fix(capture): preserve PhotoStage preview aspect on first paint

Contain-fit waits for a real parent width, uses ResizeObserver, and shrinks
width when maxH would otherwise stretch the CSS box. Pure fitDisplay is
covered by node tests.

frontend/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc --noEmit && vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "node --import tsx --test src/**/*.test.ts"
   },
   "dependencies": {
     "idb-keyval": "^6.3.0",
@@ -15,6 +16,7 @@
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^3.1.2",
     "svelte-preprocess": "^5.1.4",
+    "tsx": "^4.23.13",
     "typescript": "^5.7.3",
     "vite": "^5.4.19"
   }