refactor: swap inline mirror for published @such-software/smirk-dapp-api

The runtime that installs window.smirk in v0.3.0+ iframe context
is now available as a published npm package, so smirk.cash can
import it directly instead of carrying a synced local copy. Same
behaviour, less maintenance burden.

- npm install @such-software/smirk-dapp-api@^0.3.0
- SmirkBridgeBootstrap imports installSmirkPageApi from the package
- Local mirror deleted (src/lib/smirk-iframe-bridge.ts)

package.json

@@ -9,6 +9,7 @@
     "lint": "eslint"
   },
   "dependencies": {
+    "@such-software/smirk-dapp-api": "^0.3.0",
     "next": "15.3.6",
     "react": "^18.3.1",
     "react-dom": "^18.3.1"