Drive invitation poster palette from the linked theme's signature colours

[lucaslyl]

summary:

The invitation-poster prompt only fed the linked theme's name and description
text to the image model, so the generated poster's colours were left to the
model's guess and did not follow the theme.

Derive concrete colours from the linked theme's computed cssVariables and
pin them in the prompt:

• background ground = theme --primary (its signature/chrome hue; e.g. a red
  theme now yields a red poster), falling back to --navy / --ink-2
• lettering = theme --primary-foreground (its on-primary contrast colour),
  falling back through --gold-soft / --navy-ink / --paper / --background
• decorative motif + accents = theme --accent (falling back to --gold /
  --acc)

The prompt now states these exact hex values and instructs the model not to
substitute other colours. When no theme is linked, the previous warm-white
default is unchanged.