[mstomar125]

Refs #3864

Defence in depth for the chat HTML sanitiser (the CSP already blocks these at load time in the shipped app).

Changes

• poster, background and cite are now covered. cite is held to the link allow-list; poster and background fetch on their own, so they follow the media rule below.
• Inline style is removed (DOMPurify FORBID_ATTR) rather than filtered. Chat markdown never emits inline style, so there is nothing to keep, and a filter for url(), image-set() and position:fixed would be a second parser to get wrong.
• Attributes the browser fetches without a click (src, poster, background) now accept only a relative reference. img (and poster/background) may also carry data:image/(png|jpeg|gif|webp). Remote https:// on img, audio, video, source and track is dropped. href behaviour is unchanged.
• href / xlink:href on anything other than a and area (svg image, feImage, use) is fetched by the browser, so it follows the same relative / data:image rule as src. Anchors keep the link allow-list.
• <style> elements are forbidden (FORBID_TAGS), including inside svg; app styles live in the Svelte component, not in message HTML.
• action / formaction are dropped.
• daemon_setup.rs: the "bundle changed while it was being installed" re-check after the rename is now ensure_bundle_unchanged, with a unit test (changed size, vanished bundle, unchanged passes).

User-visible change

Remote markdown images (![alt](https://...)) and remote <img> in AI replies no longer render. This is the intended privacy trade: no tracking pixel or IP leak from a message.

Tests

Browser vitest (real Chromium + DOMPurify) for each point; each was mutation-checked (cite, poster/background, style, remote-src, data-image-on-non-img) and fails without its change. The Rust test fails when the check is stubbed out.

Not done

• ping, longdesc and data attributes are unchanged (srcset was already dropped).
• The race itself (bundle replaced mid-install) is not reproduced end to end; only the check is tested.

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