## Summary
- Scoreboard's render page/URL now carries the operator's background selection explicitly via `?bgcolor=<color>`, validated against this component's own known `SCOREBOARD_BACKGROUND_OPTIONS` set (`resolveValidatedBgcolor`) -- the same convention overlays.uno uses, resolved the same way sink's own `with_chroma_key_background` now resolves it for every provider (see sink#124).
- Share Tab's own copy-link URL embeds the current selection too, instead of relying solely on the session's live persisted config -- what you copy is what plays. `"transparent"` has no color value, so that's the one case the param is simply omitted for (no `?bgcolor=transparent`).
- `bgcolorOverride` (the render page's validated URL param) takes precedence over the persisted `config.appearance.outputBackground` when present -- this is what lets the same session be embedded with a *different* background in two places at once: an operator's own direct-OBS copy stays on their persisted default, while sink's own `CompositionSource` URL construction forces its fixed key color independent of that.
- Reverts match-info's earlier `mode=chromakey` wiring -- match-info has no background-selection concept of its own yet, and there may be in-flight teammate work here; leaving it untouched for now.