Design-sync back-mirror of the #404 weight UI (PR #462) into the Claude Design
prototype so repo + Claude Design stay byte-identical (docs/design-sync.md step 7).
Pushed to the ChicoryTV Design System project via DesignSync in the same session.
The prototype's EditItemRow now shows a per-source Weight input (1..1000) + computed
% share, and the editor gains a "Reset to fair share" footer — a faithful mockup of
the shipped screen. Prototype-only (design-system/), no shipped code.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>