fix(canvas): block browser pinch-zoom inside breakpoint frame iframes

[DavidBabinec]

Symptom

Hovering a breakpoint frame's toolbar (e.g. Switch to Tablet breakpoint) and pinch-zooming out on a trackpad zooms the entire admin page — which is supposed to be locked.

Root cause

AdminZoomGuard cancels pinch / ctrl-wheel / Safari gesture* browser zoom, but it's installed only on the parent admin document (main.tsx). Each canvas breakpoint frame renders inside its own iframe document, and wheel/gesture events that originate inside an iframe never cross the boundary to the parent — so the parent guard never sees them.

The design frame had an ad-hoc wheel forwarder (IframeFrameSurface) that happened to preventDefault() wheel events (to drive canvas pan), which masked the issue for plain ctrl-wheel in Chrome — but it never handled gesturestart/gesturechange or multi-touch pinch, and live-preview frames have no forwarder at all. So pinch over a frame escaped to the browser and zoomed the page.

Fix

Install the existing installAdminZoomGuard inside every frame document (design and live), where IframeFrameSurface already owns iframeDoc. The wheel forwarder still routes pan/zoom to the canvas; the guard only cancels the browser's native zoom default. One small effect, reusing the shared guard — no new mechanism.

Verification

• bun run build ✓ · bun run lint ✓ · bun test ✓ — 5436 pass, 0 fail
• New test (iframeFrameZoomGuard.test.tsx) renders a real CanvasRoot and asserts the frame iframe document cancels ctrl/meta-wheel, gesturestart, gesturechange, and multi-touch pinch, while leaving single-finger scroll alone.
• Live in the running editor (against the real site): all three frames (mobile/tablet/desktop) report ctrlWheelPrevented=true and gesturestartPrevented=true inside their iframe documents (the gesturestart block is the new behavior — before the fix only wheel was cancelled, by the forwarder).

Note: the pre-push hook surfaces 3 react-doctor warnings on IframeFrameSurface.tsx (the forwardRef import, the wheel forwarder's listener, the deliberately same-origin/unsandboxed canvas iframe). All are pre-existing and unrelated to this change; they appear only because the file is in the diff. bun run lint (the CI gate) passes.

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