Migrate build system to esbuild and fix e2e code coverage

[chensation]

Summary

After the Angular 19 update, the build system and e2e code coverage need attention. This issue tracks migrating from the legacy webpack-based builder to esbuild and restoring Cypress e2e code coverage.

Current Problems

• E2e coverage disabled: CYPRESS_COVERAGE=false was added to CI because the production build has no Istanbul instrumentation, causing OOM crashes and Invalid string length errors in the coverage report afterAll hook.
• Legacy builder: angular.json uses @angular-devkit/build-angular:browser (webpack). Angular 19+ defaults to the esbuild-based application builder.
• Dead dependencies: istanbul-instrumenter-loader is referenced in cypress/coverage.webpack.js but not installed. ngx-build-plus is used only for the serve builder.

Tasks

1. Migrate to esbuild application builder

• Change builder from @angular-devkit/build-angular:browser to @angular-devkit/build-angular:application in angular.json
• Update build config for esbuild compatibility (main → browser, outputPath structure, polyfills format)
• Remove ngx-build-plus dependency and custom webpack serve config
• Validate dev server, proxy config, and all build configurations still work
• Validate CI pipeline (lint, unit tests, e2e) passes

2. Fix e2e code coverage

• Find an esbuild-compatible approach for Istanbul instrumentation (e.g., custom esbuild plugin or babel-plugin-istanbul via a custom plugin)
• Update cypress/coverage.webpack.js or replace with esbuild equivalent
• Add a CI-specific build config with instrumentation, source maps enabled, and optimization off
• Set NODE_OPTIONS: --max-old-space-size=4096 for the e2e CI step
• Remove CYPRESS_COVERAGE: false from build.yml once coverage works
• Verify combined.js merge script and nyc report still function correctly

3. Fix tree component tests after build migration

• Fix tree tests that break after the build system migration to esbuild

Context

• Angular 19 application builder uses esbuild + Vite dev server
• The webpack-based browser builder is deprecated and will be removed in a future Angular version
• ngx-build-plus and istanbul-instrumenter-loader do not support esbuild