Give the dashboard its own favicon set

[MaciejZiel]

Replaces the blue square data-URI icon with stacked queue bars (blue accent) on the shared dark rounded tile used across the other sites. Original artwork, no third-party icons.

• favicon.svg, favicon.ico (16/32/48), favicon-32x32.png and a 180 px apple-touch-icon in the admin static dir
• dashboard head links them and adds light/dark theme-color tags matching the page backgrounds
• the admin app answers /favicon.ico (image/x-icon, one-day cache); the Termux nginx snippet serves /favicon.ico and /apple-touch-icon.png at the host root, where browsers look first
• a test checks the links and the content type of every icon path