[1.0] Backport CVE-2026-22254: Sanitize SVGs when uploaded to the theme assets

[vulgraph]

Hello WinterCMS team,

Following up on the SVG sanitization fix from upstream — CVE-2026-22254 was applied to develop/1.2, but the 1.0 LTS line is currently missing it. This PR cherry-picks the upstream fix verbatim onto 1.0; the patch applied without conflicts and the (cherry picked from commit 8a7f74b004) trailer is preserved.

Reference:

• Fix commit: 8a7f74b
• CVE: CVE-2026-22254 (stored XSS via SVG theme assets)

If 1.0 is no longer receiving security fixes, feel free to close — wanted to surface it in case the omission was inadvertent.

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Backport of upstream fix for CVE-2026-22254 to 1.0.

• Upstream commit: 8a7f74b004 — Sanitize SVGs when uploaded to the theme assets
• Cherry-picked with git cherry-pick -x (original author preserved).

Apply was clean against the current tip of the target branch. No code changes on top of the upstream fix.

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