Update Angular from 20 to 21

[kyasbal]

This PR updates the frontend dependency from Angular 20 to 21.

NOTE: All of the changes except listed below are made by the automatic code updator run by ng update @angular/core@21 @angular/cli@21.

Manually changed files:

• ./scripts/generate-angular-json.sh: Removed unused property after updating the angular build system
  (https://angular.jp/tools/cli/build-system-migration)
• web/src/app/services/data-loader.service.ts: Changed type notation in method to fix the compilation error after the type changes on Angular side.

[gemini-code-assist]

Summary of Changes

Hello @kyasbal, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a significant upgrade of the frontend application's Angular framework from version 20 to 21. This update brings the project in line with the latest Angular standards, incorporating new features and performance improvements. The bulk of the changes are automated, reflecting the smooth migration path provided by the Angular CLI, complemented by targeted manual fixes to ensure full compatibility and leverage new syntax.

Highlights

• Angular Version Upgrade: The primary change is an upgrade of the Angular framework from version 20 to 21, along with corresponding updates to related Angular packages and dependencies.
• Automated Code Updates: Most of the changes in this pull request were automatically generated by the Angular CLI's ng update @angular/core@21 @angular/cli@21 command, ensuring compatibility with the new Angular version.
• Manual Adjustments for Compatibility: Specific manual changes were made to address breaking changes, including removing an unused property in a shell script (scripts/generate-angular-json.sh) and fixing a type notation in web/src/app/services/data-loader.service.ts.
• Migration to New Control Flow Syntax: Angular template files have been updated to use the new built-in control flow syntax (@if, @for) instead of the older structural directives (*ngIf, *ngFor), improving readability and performance.
• Build System and Test Environment Updates: The Angular build builder was migrated from @angular-devkit/build-angular:browser to @angular/build:application, and test environment setups were adjusted to include provideZoneChangeDetection for better testing practices.

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[gemini-code-assist]

Code Review

This pull request updates Angular from version 20 to 21. The majority of the changes are automated migrations performed by ng update, which include updating dependencies, build configurations, and adopting the new control flow syntax in templates. These automated changes appear to be correct. I've added a few comments: one high-severity comment regarding a manual type change that seems incorrect, and several medium-severity comments about misplaced imports in test files, likely an artifact of the update tool. Overall, the update is well-executed, pending the resolution of the noted issues.

[kyasbal]

Closes #406