webui: add HTML/JS preview support to MarkdownContent with sandboxed iframe (#16757)

webui: add HTML/JS preview support to MarkdownContent with sandboxed iframe (#16757)

* webui: add HTML/JS preview support to MarkdownContent with sandboxed iframe dialog

Extended MarkdownContent to flag previewable code languages,
add a preview button alongside copy controls, manage preview
dialog state, and share styling for the new button group

Introduced CodePreviewDialog.svelte, a sandboxed iframe modal
for rendering HTML/JS previews with consistent dialog controls

* webui: fullscreen HTML preview dialog using bits-ui

* Update tools/server/webui/src/lib/components/app/misc/CodePreviewDialog.svelte

Co-authored-by: Aleksander Grygier <redacted>
* Update tools/server/webui/src/lib/components/app/misc/MarkdownContent.svelte

Co-authored-by: Aleksander Grygier <redacted>
* webui: pedantic style tweak for CodePreviewDialog close button

* webui: remove overengineered preview language logic

* chore: update webui static build

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Co-authored-by: Aleksander Grygier <redacted>