chore(brand): align app-store hex to canonical cobalt #21468B

The icon hex was filled with the legacy '#4376FC' (the
conduction-design-tokens 'Conduction Cobalt'). The design-system
has retired this in preview/colors.html with the caption

> Our legacy blue (#4376FC) read as 'cloud / SaaS', light, frisky,
> lacking weight. Cobalt is the official blue of the Dutch flag,
> Dutch by birthright, dark enough to feel serious, light enough
> to stay blue, and AAA-contrast on white so it can carry body
> text.

Swap to #21468B (--c-blue-cobalt in tokens.css) so the app-store
hex matches the navbar wordmark and the rest of the brand chrome.
Fleet sweep 2026-05-13.

Author: rubenvdlinde