feat(ai-create): let users choose brand colors or free AI colors for images

[dantaspaulo]

Problem

The AI image pipeline already threads an applyBrandVisuals flag all the way
through TemplateContext → PostImagePipeline → TemplateImageGenerator (when
off, the AI background is generated without the workspace brand color,
background, text color or brand description). But the flag was hardcoded to
true at the dispatch site, so users had no way to opt out — every generated
image was forced onto the workspace brand palette.

Change

Expose the choice in the create wizard as a "Brand colors" / "Let AI decide"
toggle, shown only when images are actually generated. The flag flows:

AiPostWizard (apply_brand_visuals)
  → StartPostCreationRequest (validation)
  → PostAiCreateController@start (dispatch)
  → StreamPostCreation (job)
  → TemplateContext.applyBrandVisuals
  → PostImagePipeline.forSingle/forCarousel
  → TemplateImageGenerator

It defaults to true, so behavior is unchanged when the field is absent (API
clients, existing callers). Only the AI-image templates (image_card /
carousel) honor the flag — tweet cards are brand-colored by design and their
pipeline path doesn't take applyBrandVisuals.

• StreamPostCreation: new applyBrandVisuals constructor arg, passed into
  TemplateContext.
• PostAiCreateController@start: reads apply_brand_visuals and forwards it.
• StartPostCreationRequest: apply_brand_visuals → ['sometimes', 'boolean'].
• AiPostWizard.vue: the toggle + sends the flag.
• lang/{en,es,pt-BR}/posts.php: brand_colors_* strings.

Tests

PostAiCreateTest: the job carries applyBrandVisuals === false when the user
opts out, and defaults to true when the field is omitted.