[SandObserver]

Summary

• Seed the backup placeholder drawing from a per-card seed instead of Math.random, so both faces of a card render an identical pattern and the poll-time redraw repeats it.
• Give the back canvas the same reserved height as the front canvas, so the row plan is identical on both faces and the opaque name zone covers the extra rows.

Testing
All green, Docker smoke build left to CI. No new coverage: the change is inline widget drawing code with no testable module surface.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.