Vendor frontend JS into javascript/; drop the prestartup copy dance

The viewer JS/HTML/bundles are now committed in javascript/ (declared via
pyproject [tool.comfy] web = "javascript") instead of being copied from the
shared comfy-3d-viewers pip package at prestartup. Consequences:

- requirements.txt is now just comfy-env (host env installs nothing else,
  per the never-touch-the-host-env principle)
- prestartup_script.py no longer copies viewers or web_src; it keeps only
  setup_env() and the input-asset copies
- WEB_DIRECTORY attribute removed; the pyproject declaration is the single
  source (served at /extensions/comfyui-geometrypack/, keyed by project name)
- web_src/ deleted; its 6 widgets now live in javascript/js/ as the tracked
  source, edited directly
- viewer_pbr.html hardcoded-fallback URL updated to the new mount name

Iframe URLs auto-detect the mount from import.meta.url, so the folder rename
and mount-name change are transparent at runtime.

javascript/js/viewer/utils/MessageHandler.js

@@ -0,0 +1,100 @@
+/**
+ * PostMessage communication utilities for VTK viewers
+ */
+
+/**
+ * Message types used by the viewer system
+ */
+export const MessageTypes = {
+    // Incoming messages (from parent)
+    LOAD_MESH: 'LOAD_MESH',
+    LOAD_DUAL_MESH: 'LOAD_DUAL_MESH',
+
+    // Outgoing messages (to parent)
+    MESH_LOADED: 'MESH_LOADED',
+    MESH_ERROR: 'MESH_ERROR',
+    SCREENSHOT: 'SCREENSHOT'
+};
+
+/**
+ * Send a message to the parent window
+ * @param {string} type - Message type
+ * @param {Object} data - Additional data to send
+ */
+export function sendToParent(type, data = {}) {
+    if (window.parent && window.parent !== window) {
+        window.parent.postMessage({
+            type,
+            ...data
+        }, '*');
+    }
+}
+
+/**
+ * Send an error message to the parent window
+ * @param {string|Error} error - Error message or Error object
+ */
+export function sendError(error) {
+    const message = error instanceof Error ? error.message : error;
+    sendToParent(MessageTypes.MESH_ERROR, { error: message });
+}
+
+/**
+ * Send a screenshot to the parent window
+ * @param {string} imageDataUrl - Base64 PNG data URL
+ */
+export function sendScreenshot(imageDataUrl) {
+    sendToParent(MessageTypes.SCREENSHOT, {
+        image: imageDataUrl,
+        timestamp: Date.now()
+    });
+}
+
+/**
+ * Send mesh loaded confirmation to parent
+ */
+export function sendMeshLoaded() {
+    sendToParent(MessageTypes.MESH_LOADED, { error: null });
+}
+
+/**
+ * Create a message listener for the viewer
+ * @param {Object} handlers - Object mapping message types to handler functions
+ * @returns {Function} Cleanup function to remove the listener
+ */
+export function createMessageListener(handlers) {
+    const listener = (event) => {
+        const { type } = event.data || {};
+
+        if (type && handlers[type]) {
+            handlers[type](event.data);
+        }
+    };
+
+    window.addEventListener('message', listener);
+
+    // Return cleanup function
+    return () => {
+        window.removeEventListener('message', listener);
+    };
+}
+
+/**
+ * Extract filename from a filepath URL
+ * @param {string} filepath - URL or path to extract filename from
+ * @returns {string} Extracted filename
+ */
+export function extractFilename(filepath) {
+    const urlParams = new URLSearchParams(filepath.split('?')[1] || '');
+    return urlParams.get('filename') || filepath.split('/').pop().split('?')[0];
+}
+
+export default {
+    MessageTypes,
+    sendToParent,
+    sendError,
+    sendScreenshot,
+    sendMeshLoaded,
+    createMessageListener,
+    extractFilename
+};