docs: add branded icon assets and align docs color tokens

[sjquant]

Why

• give QuickThumb a repo-specific app icon that reflects the thumbnail-card plus code-first identity
• make the documentation palette match the new rgb(77,137,251) brand direction instead of the older cyan/black split
• publish a complete favicon and touch-icon set that the docs site can actually serve

Changes

• generated a new blue brand icon with tighter two-card overlap and exported the icon set under assets/brand
• published the docs-facing favicon, Apple touch icon, and web manifest assets and wired them into MkDocs
• refreshed the Material for MkDocs color tokens in docs/stylesheets/extra.css so light/dark surfaces, links, code blocks, and primary chrome align with the new icon palette