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/tsconfig.json

@@ -12,5 +12,6 @@
     "verbatimModuleSyntax": false,
     "types": ["vite/client"]
   },
-  "include": ["src/**/*.ts", "src/**/*.svelte", "vite.config.ts"]
+  "include": ["src/**/*.ts", "src/**/*.svelte", "vite.config.ts"],
+  "exclude": ["src/**/*.test.ts"]
 }