The image pipeline already threads `applyBrandVisuals` through
`TemplateContext` -> `PostImagePipeline` -> `TemplateImageGenerator`, but it was
hardcoded to `true` at the dispatch site, so generated images always used the
workspace brand palette with no way to opt out.
Expose the choice in the create wizard: a "Brand colors" / "Let AI decide"
toggle (shown only when images are generated). The flag flows
front -> `StartPostCreationRequest` (`apply_brand_visuals`) ->
`PostAiCreateController@start` -> `StreamPostCreation` -> `TemplateContext`,
defaulting to `true` so existing behavior is unchanged when the field is absent.
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