Constant HOST_API_BRIDGE_SCRIPT 

pub const HOST_API_BRIDGE_SCRIPT: &str = r#"
(function() {
    'use strict';
    if (window.__hostApiBridge) { return; }
    window.__hostApiBridge = true;
    window.__HOST_WEBVIEW_MARK__ = true;
    var ch = new MessageChannel();
    window.__HOST_API_PORT__ = ch.port2;
    ch.port2.start();
    var port1 = ch.port1;
    port1.start();
    port1.onmessage = function(ev) {
        var data = ev.data;
        if (!data) { console.warn('[host-bridge] data is falsy, dropping'); return; }
        var bytes;
        if (data instanceof Uint8Array) { bytes = data; }
        else if (data instanceof ArrayBuffer) { bytes = new Uint8Array(data); }
        else if (ArrayBuffer.isView(data)) { bytes = new Uint8Array(data.buffer, data.byteOffset, data.byteLength); }
        else { console.warn('[host-bridge] unknown data type: ' + typeof data + ' constructor=' + (data.constructor ? data.constructor.name : '?') + ', dropping'); return; }
        var binary = '';
        for (var i = 0; i < bytes.length; i++) { binary += String.fromCharCode(bytes[i]); }
        try {
            window.webkit.messageHandlers.hostApi.postMessage(btoa(binary));
        } catch(e) {
            console.error('[host-bridge] postMessage to hostApi FAILED:', e.message);
        }
    };
    window.__hostApiReply = function(b64) {
        try {
            var binary = atob(b64);
            var bytes = new Uint8Array(binary.length);
            for (var i = 0; i < binary.length; i++) { bytes[i] = binary.charCodeAt(i); }
            port1.postMessage(bytes);
        } catch(e) { console.error('[host-bridge] reply failed:', e.message); }
    };
})();
"#;

JavaScript injected before the Polkadot app loads. Sets up:

1. window.__HOST_WEBVIEW_MARK__ = true — SDK webview detection
2. MessageChannel with port2 as window.__HOST_API_PORT__
3. Binary message forwarding between port1 and native (base64)