feat(*): re-implementing theme handler on better base

[ChronosSF]

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[Copilot]

🟡 Changes recommended

There is a confirmed runtime bug in resolveMode() (optional chaining misuse) and an avoidable performance regression due to an unconditional dynamic import.

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Pull request overview

This PR adds a message-driven theme handler so sample pages can be re-themed dynamically when embedded in the docs site iframe, by retargeting theme <link data-ig-theme> stylesheets and syncing Ignite UI Web Components’ shadow-style theming.

Changes:

• Introduces initSampleThemeListener() to validate postMessage senders and apply theme/mode updates (including “system” mode).
• Retargets existing theme <link data-ig-theme> URLs to swap variant/theme CSS at runtime, then calls configureTheme() to re-adopt shadow styles.
• Wires the theme listener into the sample page layout so it runs on every sample page.

File summaries

File	Description
src/utils/sample-theme.ts	Adds the postMessage listener, origin validation, CSS link retargeting, and shadow-style re-adoption via configureTheme().
src/layouts/SampleLayout.astro	Initializes the sample theme listener on sample pages so docs can drive theme changes in an iframe.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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