feat(loading-spinner): support loading text below spinner

[chintankavathia]

Summary

Adds a loadingText input to SiLoadingSpinnerDirective and SiLoadingSpinnerComponent that displays an optional translatable text label below the spinner animation.

Motivation

Our Loading pattern already documents and visually shows a spinner with accompanying text, and the best practices explicitly recommend:

"Consider complementing the loading indicators with a brief text for context."

However, until now there was no built-in way to render that text — consumers had to implement custom overlays or workarounds to achieve what the pattern already prescribes. This gap between documented guidance and available API caused inconsistent implementations across teams.

What's included

• New loadingText input on both the directive ([siLoading]) and the component (si-loading-spinner), accepting a TranslatableString for i18n support.
• Layout adjustment — the spinner container switches to flex-direction: column so text renders centered below the SVG.
• Runtime reactivity — changing loadingText while the spinner is active updates the displayed text immediately via setInput.
• Unit test verifying text is rendered in the DOM when provided.
• Live example (si-loading-spinner-text) demonstrating a realistic full-page loading state with contextual text ("Loading Dashboard…").

Usage

<div [siLoading]="isLoading" loadingText="Loading Dashboard...">
  <!-- content -->
</div>

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Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request introduces an optional loadingText input to the SiLoadingSpinnerComponent and SiLoadingSpinnerDirective, allowing translatable text to be displayed below the spinner. The changes include styling updates to support a vertical layout, logic to pass the text through the directive's portal, and a new example page. Review feedback focused on aligning user-facing strings in tests and examples with UX writing guidelines, specifically regarding the use of the ellipsis character, sentence casing, and avoiding polite fillers like "please".

[chintankavathia]

@panch1739 here is the example https://d33c9dcnqinn2a.cloudfront.net/pr-1957/pages/element-examples/#/overview/si-loading-spinner/si-loading-spinner-text?q=loadin

[spliffone]

Could we adjust the the existing si-loading-spinner and si-loading-spinner-directive examples instead of a new example?