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/src/wasm.d.ts

@@ -0,0 +1,24 @@
+/// Type declarations for the idfoto WASM module produced by wasm-pack.
+
+// Ambient module declarations for the WASM glue code.
+// The module specifier must exactly match what's used in import statements.
+
+declare module 'idfoto-wasm' {
+  export default function init(input?: string | URL): Promise<void>;
+  export function derive_master_key(
+    passphrase: string,
+    image_secret: Uint8Array,
+    salt: Uint8Array,
+    params_json: string,
+  ): Uint8Array;
+  export function encrypt(plaintext: Uint8Array, key: Uint8Array): Uint8Array;
+  export function decrypt(ciphertext: Uint8Array, key: Uint8Array): Uint8Array;
+  export function extract_image_secret(jpeg_bytes: Uint8Array): Uint8Array;
+  export function encrypt_entry(entry_json: string, key: Uint8Array): Uint8Array;
+  export function decrypt_entry(ciphertext: Uint8Array, key: Uint8Array): string;
+  export function encrypt_manifest(manifest_json: string, key: Uint8Array): Uint8Array;
+  export function decrypt_manifest(ciphertext: Uint8Array, key: Uint8Array): string;
+  export function generate_totp(secret_base32: string, timestamp_secs: bigint): string;
+  export function generate_password(length: number): string;
+  export function generate_entry_id(): string;
+}