### Overview In a content template editor, after previewing the template and returning to the editor, the preview frame keeps showing a **stale preview snapshot** instead of the live editor content. Subsequent edits — for example linking a component prop to an entity field, or changing a text field — update the form and the model, but the preview frame does not reflect them. The edits appear to have no effect until an unrelated action forces a fresh render. The editor frame's source is selected as `snapshotHTML || html`: the preview snapshot takes precedence over the live editor html whenever it is set. The snapshot is populated by the preview and is only cleared on the `canvas_page` layout path and, for **language** previews, on preview unmount. A plain (non-language) content template preview registers no such cleanup, and the content template editor's own render paths (template layout load, component update) never clear the snapshot — so a leftover snapshot masks live editor updates. The failure is **deterministic** for a plain full-page preview of a content template (nothing clears the snapshot afterwards). For a **language** preview it is intermittent — the unmount cleanup runs, but a late preview response can re-populate the snapshot after it. The latter is the root cause of the flaky Playwright test `Templates - General › Add templates to page`, where the preview `<h1>` is expected to reflect a field-linked value but keeps showing the component's default heading. ### Steps to reproduce 1. Create a content type (e.g. Article) with at least one node. 2. In Canvas, create a "Full content" content template for it containing a component with a text prop (e.g. a hero heading). 3. Open the template in the editor, click **Preview** to open the full-page preview, then **Exit Preview** to return to the editor. 4. Edit the component — for example link its heading prop to the entity's Title field, or type into a text field. 5. The preview frame stays frozen on the previewed snapshot and does not reflect the edit. ### Proposed resolution Clear the preview snapshot when the content template editor establishes its own live content, so the live html wins over a leftover snapshot. This mirrors the existing snapshot clear on the `canvas_page` layout path added in #3591684; the content template layout and the component-update paths were missed and need the same treatment. ### Remaining tasks - Clear the stale snapshot on the content template editor's layout and component-update paths. - Confirm the previously flaky `Add templates to page` test is stable. ### User interface changes None. The preview now reflects live edits instead of a stale snapshot.