[upstream #16596] Sources caption, PromptForm backdrop and Insights picker popover still carry palette or dark: colours #222

Left out of LibreChat-AI#16596 (media overlay roles, themable C6) because no existing role keeps their default pixels, so each needs a small decision before it moves:

• client/src/components/Web/Sources.tsx:159: the image caption is bg-gray-900/80 text-white over the source thumbnail. It is a media overlay, but surface-media-overlay is black and the caption is #0d0d0d at 80%. Either accept the one-step shift (at most 10 levels after compositing) and use bg-surface-media-overlay/80 text-text-on-media, or keep #0d0d0d as the media role's default.
• client/src/components/Prompts/forms/PromptForm.tsx:585: the mobile side-panel backdrop is bg-black/20, a page scrim rather than a media overlay. It belongs with the mobile drawer scrim work in [upstream #16492] Mobile drawer scrim and edge on the theme scrim role (after #16248) #199 (bg-scrim or a lighter page-scrim role), which changes the default in light mode.
• client/src/components/Insights/InsightsView.tsx:772: the agent picker popover is bg-surface-primary ... dark:bg-chart-widget-surface on a MultiSelect. Swapping it for bg-chart-widget-surface (identical pixels in every bundled theme) is a same-count no-restyle swap the design-lint ratchet rejects; MultiSelect needs a popover surface prop or variant first.

Still owned by open PRs and tracked in #195: Artifacts.tsx (LibreChat-AI#15911) and AgentConfig.tsx (LibreChat-AI#16523).