feat(theme): preview-first Theme Studio + arbitrary-color hex shade overrides

- Add PanelThemeStudio editor (BaseColorRail | ComponentList | LivePreview | ComponentInspector)
- ChannelEditorModal: accordion of 4 channels, base color + per-shade overrides via categorized hex picker
- Per-token hex overrides at bg-{channel}-{level} derive at slot's shade level (ThemeModel)
- LivePreview shows component scenarios (Alone / Inside Panel / Inside Card / With Siblings)
- ComponentList covers ~70 dash-react primitives with per-component token inspector
- ThemeWrapper writes active theme cssVars to :root so hex themes render app-wide
- cssValueFor resolves hex channels to literal hex (theme tiles render without :root)
- Remove legacy PanelTheme editor + dead test override (bg-primary-very-dark: bg-black)
- Update ThemeModel tests + mock for new cssValueFor behavior