Angular 21 support

[jogaj]

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

What is the new behavior?

• Added support for Angular 21
• Replaced docSpa with storybook
• Removed support for Angular 18
• Updated cypress tests to run against storybook

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...
Removed support for angular 18

Other information:

---

Note

Medium Risk
Moderate risk due to build/CI pipeline changes (Node upgrade, Storybook build+deploy, custom webpack builder) and updated Cypress test harness, which could break publishing or CI if configuration is off.

Overview
Adds Angular 21 support for @swimlane/ngx-dnd and documents the release as 12.0.0-alpha.0, including dropping Angular 18 support.

Migrates the demo/docs surface to Storybook: adds Storybook build targets in angular.json, adjusts ESLint to lint Storybook/stories with a dedicated tsconfig.storybook.json, and updates Cypress E2E tests to run against Storybook’s iframe.html via a new cy.visitStory helper.

Updates CI to build Storybook and deploy the generated dist/storybook/ngx-dnd to gh-pages, while also upgrading the GitHub Actions Node runtime to 22.16.

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