Redesign the demo page

[cobanov]

The demo worked and looked like a default: system font, a card on a grey background, controls with no hierarchy. It now speaks the same language as the other cobanov.dev pages.

• Film-base black, one accent. The accent is the subject rather than a brand colour: a surface facing the camera encodes as rgb(128, 128, 255), so the page runs on the colour every normal map is mostly made of, and the mark is a normal-mapped sphere.
• Geist for reading, JetBrains Mono for anything measured. Pixel counts, milliseconds, option names.
• Sticky masthead, film grain, base-8 spacing, a named type scale, focus-visible rings, a skip link.
• Each panel carries its own reading: the source its pixel size, the output its conversion time. The status line is left for messages.
• A full head: description, canonical, OpenGraph and Twitter cards, structured data, an SVG favicon, and an OG card built from the Mars pair.
• _headers: Pages caches assets for four hours by default, which is wrong for the two files carrying the page's behaviour, plus a CSP naming the three hosts the page may reach (jsdelivr for ONNX Runtime, Google Fonts, Hugging Face for the depth model). Verified live: no violations, and the model fetch still returns 206.
• The sample is an 8-bit depth map, so the page opens at sigma 2 rather than showing its quantisation rings as a first impression. The note beside it explains why they appear.

Deployed and checked on depth2normal.cobanov.dev: Geist loads, the conversion runs in about 300 ms, the readouts fill in.