✨ Mailer: central brand values via config.mailer.brand

[PierreBrisorgueil]

What

• Add mailer.brand with these fields:
  • name, falling back to app.title
  • url, falling back to app.url || getBaseUrl()
  • contact, falling back to app.contact
  • logoUrl, primaryColor, textColor, mutedColor, fontFamily, signature, footerText
  • links as a { label: url } object (arrays replace on deep merge)
• Export a getBrand() getter as the single source of the resolved values.
• render injects brand, plus the legacy appName (= brand.name) and appContact (= brand.contact). Brand wins over caller-passed values for those two keys only.

Acceptance criteria

• With no brand config, the output is identical to today's.
• Setting mailer.brand.name changes {{appName}} in every mail, including templates that exist only in a downstream.
• DEVKIT_NODE_mailer_brand_<field> env vars set brand values.
• A partial brand override keeps the other defaults.
• All brand values render escaped.

Tests

• Fallbacks with an empty brand.
• An override from a mocked config shaped like a downstream override.
• Brand winning over a caller-passed appName.
• A deepMerge check with a partial brand override.

Depends on: resolver (render).