furl.html: redesign decrypt page (Atmosphere Pro styling) + a11y/UX fixes

[LilianGRD]

Design refresh of the web/furl.html download page, styled after the atSign Atmosphere Pro app. The decryption logic is unchanged — only presentation plus a few DOM/UX fixes. Still one self-contained file; wasm-crypto.js untouched.

Design

• Brand orange (#FF6633), outlined atSign "@" logo, gray "tile" panels, Space-Mono-style section labels, solid orange CTA
• First-class dark mode (prefers-color-scheme) + mobile layout, inline SVG icons (no emoji)

Fixes

• File info now fills the name/size elements via textContent (was one innerHTML write that left size empty and could inject HTML from the server-provided filename)
• Button label, focus rings and input borders now meet WCAG AA contrast
• Progress exposed to screen readers (role=progressbar/aria-valuenow); errors announced assertively
• Error no longer prints the internal storage URL
• Corrected PIN hint to "9 characters, case-sensitive" (the alphabet has no special characters)

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