Favicon, font, and background color for site branding

Site branding settings gain three fields, alongside the existing logo and brand color:

• Favicon: PNG, JPEG, WebP, SVG, or ICO, up to 2 MB. Shows as the browser tab icon on the public agent page.
• Font: a curated list of 20 Google Fonts (Roboto, Inter, Poppins, and others).
• Background color: a hex color for the assistant's page background, white by default.

All three are optional, independent, and applied wherever a site's branding already showed up:

• On the playground, only the chat panel picks up the site's background, logo, font, and brand color; the dashboard sidebar and header keep their own light/dark theme.
• On the public agent page (/a/{token}), branding applies to the whole page. The page also now renders in a permanent light theme, regardless of a shopper's OS or browser dark-mode setting: it's a merchant-branded surface, not a themed dashboard page.

Buttons that use a site's brand color as their background now pick black or white text automatically, whichever is more readable against that color, so a light or dark brand color both stay legible.