Resolves the AiPostWizard conflict and completes the feature:
- i18n parity: brand_colors_label + brand_colors_description in all 15 locales
(was en/es/pt-BR only, which broke LocalizationParityTest).
- Reworked the two-button toggle into a Switch with an explanatory description
(matches the settings Switch/card pattern).
- Only shown for templates that honor the flag: added appliesBrandVisuals() to
the AiContentTemplate contract (ImageCard=true, tweet cards=false), exposed as
applies_brand_visuals in the create-page DTO, and gated the toggle on it — so
it no longer appears (as a no-op) for tweet-card styles.
- Tests: TemplateContractTest covers appliesBrandVisuals for all templates.
Replace the 'single'/'carousel' string format threaded through the AI
generation path with a typed GeneratorFormat enum (PostContentGenerator,
PostContentHumanizer, PostContentStreamer, StreamPostCreation,
RunGenerateNode, deriveFormat).
Remove generatorFormat() from AiContentTemplate and the three templates —
it conflated style and format (returning 'tweet_card'). The tweet-card
check now lives on ContentStyle via isTweetCard(), and the humanizer skip
on ContentStyle::humanizes(), so the style declares its own behaviour
instead of callers comparing magic strings.
