fix(Headless): #3591830 Restore browser-based component sync for Canvas Headless

Bálint Klérirequested to merge
issue/canvas-3591830:3591830-restore-headless-browser-sync into 1.x

Restores browser-based component sync so Canvas can reach local frontends. Adds assertion-bound CORS across all four SDK integrations and fails fast when CANVAS_SITE_URL is missing.

Closes #3591830 (closed). Authored with an AI coding agent.

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