Title: [Button]: add loading state #791

Problem

ButtonComponent ([tedi-button]) has no loading state. A button that starts a request, for example "Load more" below a list, has to block repeated clicks while the request runs. The only built-in way is disabled, which removes the button from the tab order: keyboard focus falls to <body> and a screen-reader user loses their place (WCAG 2.4.3).

aria-disabled="true" keeps the focus, but TEDI does not style it. .tedi-button:disabled is the only disabled rule, so an aria-disabled button keeps its normal and hover colours, and each app imitates the disabled look with its own CSS.

The React library already has this: Button takes isLoading, which sets aria-disabled and aria-busy, shows a spinner, and ignores clicks while the button stays focusable (TEDI-Design-System/react#763).

Solution

Add a loading input to ButtonComponent, matching React's isLoading:

• sets aria-disabled="true" and aria-busy="true" on the host;
• applies the disabled colours (--button-main-disabled-*) and cursor: not-allowed, without hover and active styles;
• keeps the button focusable, with the visible focus outline;
• a click while loading does not reach the consumer's (click) handler, or the documentation states that the consumer ignores it.

Project

SFOS

DoD

• The component is developed using best practices, conventions, and modern frontend trends
• The component follows BEM naming conventions (if applicable)
• The component implementation matches the Figma design, including all defined states and interactions
• The component supports responsiveness and scales across breakpoints (if applicable)
• The component does not introduce runtime errors or warnings
• Linter passes
• The component is exported from its tier barrel
• The component has unit tests with at least 80% code coverage (enforced by the global Jest coverageThreshold over tedi/components, not per component)
• CodeCov passes its threshold (80% on the changed code, with 10% tolerance — a separate gate from the Jest one above)
• The component has passed code review
• The component has passed Chromatic visual review
• Storybook stories are created:
  • Cover all Figma variations
  • Include edge cases, empty/error states
• Component’s API (props, slots, events) is documented
• ZeroHeight is updated with design/development guidelines
• External dependencies used in the component are documented in Storybook and Zeroheight, with rationale
• Semantic commit format type(scope): Subject text #issue-ref is used (for semantic-release), with the issue reference required - see Commit convention
• External libraries comply with validated open source licenses
• Component is versioned correctly and ready for release if required
• Update the Angular development status on the statuses page in Zeroheight