New brand mark: favicon, app icons and share image

QRMinds' new mark is a QR finder pattern: a square ring with an inner square and a single red module in the bottom-right corner. The same mark now appears in the site header, the browser tab favicon, the icons shown when you add QRMinds to a phone, and the image shown when a link is shared.

The old logo was unreadable at small sizes and did not match the print-style design of the site. Every asset is generated from one SVG source, so it looks the same everywhere.