fix: consume error page from @salesforce/webapp-experimental only (W-21111977)

- ErrorPageRenderer: use getErrorPageTemplate() from package; remove local file read
- Remove src/templates/error-page.html (template lives in webapps proxy package)
- README: drop copy step for error-page; document package as source
- resolveDevCommand/DevServerManager: generic monorepo comments

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: ankitsinghkuntal09

README.md

@@ -123,6 +123,15 @@ sf plugins link .
 sf plugins
 ```
 
+**Build when nested in another repo (e.g. monorepo):** If `yarn build` or `npm run compile` fails with workspace name conflicts, compile manually. The error page template is consumed from `@salesforce/webapp-experimental` at runtime (W-21111977); no copy step needed.
+
+```bash
+./node_modules/.bin/tsc -p . --pretty
+sf plugins link .
+```
+
+See **[CODE_MAP.md](CODE_MAP.md)** for where AC1–AC4 and the iframe/postMessage flow live.
+
 ## Commands
 
 ### `sf webapp dev`

src/server/DevServerManager.ts

@@ -337,7 +337,7 @@ export class DevServerManager extends EventEmitter {
     this.logger.debug(`Starting dev server with command: ${this.options.command}`);
 
     // Prefer running the dev script binary directly to avoid npm workspace resolution
-    // (e.g. "multiple workspaces with the same name" when project is under a monorepo)
+    // (avoids npm workspace resolution issues when project is inside a monorepo)
     const direct = resolveDirectDevCommand(this.options.cwd, this.options.command);
     const spawnOpts: SpawnOptions = {
       cwd: this.options.cwd,

src/server/resolveDevCommand.ts

@@ -28,8 +28,8 @@ export function parseCommand(command: string): string[] {
 
 /**
  * When command is "npm run dev" (or "yarn dev"), resolve to the webapp's dev script
- * binary under node_modules/.bin to avoid npm workspace resolution (e.g. VC4 monorepo
- * "multiple workspaces with the same name" conflict).
+ * binary under node_modules/.bin to avoid npm workspace resolution issues when the
+ * project lives inside a monorepo (e.g. "multiple workspaces with the same name").
  * Returns null to fall back to the original command.
  */
 export function resolveDirectDevCommand(

src/templates/ErrorPageRenderer.ts

@@ -14,14 +14,9 @@
  * limitations under the License.
  */
 
-import { readFileSync } from 'node:fs';
-import { dirname, join } from 'node:path';
-import { fileURLToPath } from 'node:url';
 import { getErrorPageTemplate } from '@salesforce/webapp-experimental/proxy';
 import type { DevServerError } from '../config/types.js';
 
-const __dirname = dirname(fileURLToPath(import.meta.url));
-
 export type ErrorPageData = {
   status: string;
   devServerUrl: string;
@@ -32,25 +27,19 @@ export type ErrorPageData = {
 
 /**
  * Renders HTML error pages for browser display when dev server is unavailable
- * or when runtime errors occur
- *
- * Uses a single template with conditional sections for all error types
+ * or when runtime errors occur. Template is consumed from @salesforce/webapp-experimental
+ * (W-21111977: single source of truth in webapps proxy package).
  */
 export class ErrorPageRenderer {
   private template: string;
 
   public constructor() {
-    // Prefer plugin's own template (full Quick Actions: Retry, Start npm run dev, Restart, Force Kill, Proxy-only, URL)
-    const localPath = join(__dirname, 'error-page.html');
     try {
-      this.template = readFileSync(localPath, 'utf-8');
-    } catch {
-      try {
-        this.template = getErrorPageTemplate();
-      } catch {
-        console.warn('[ErrorPageRenderer] Using minimal fallback template.');
-        this.template = ErrorPageRenderer.getMinimalFallbackTemplate();
-      }
+      this.template = getErrorPageTemplate();
+    } catch (error) {
+      // eslint-disable-next-line no-console
+      console.warn('[ErrorPageRenderer] Failed to load template from package, using minimal fallback:', error);
+      this.template = ErrorPageRenderer.getMinimalFallbackTemplate();
     }
   }