feat: add extension scaffolding

Manifest, package.json, tsconfig, webpack config, popup HTML shell,
WASM type declarations, and .gitignore entries for the Chrome MV3 extension.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/tsconfig.json

@@ -0,0 +1,19 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ES2022",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "esModuleInterop": true,
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "sourceMap": true,
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "paths": {
+      "idfoto-wasm": ["./wasm/idfoto_wasm.js"]
+    },
+    "baseUrl": "."
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist", "wasm"]
+}