Redesign both screens as dark instrument UI

[MadsenDev]

Applies the visual pass from the design doc: warm near-black ground, a single
violet accent, and a split between Space Grotesk for chrome and JetBrains Mono
for anything the technician reads back.

Both roles are now dark only. The receiver's light theme is gone — it sits at a
counter beside a phone holding a camera preview, and a white page next to that
is glare. The colour scheme is declared in the shell so the first paint is never
a white flash.

The typefaces are self-hosted as Latin variable subsets (53 KB for the pair)
rather than pulled from Google Fonts. A CDN link would be blocked by the relay's
font-src 'self' anyway, and third-party resources have no business on a page
that renders scanned values.

Three things the design shows that the app did not have:

• The last scan carries its symbology and age. The age ticks inside the
  aria-live region, so it is hidden from assistive tech — otherwise a screen
  reader re-announces the scan every second.
• The phone counts delivered scans in its status bar. Counts acks, not decodes,
  for the same reason the success flash does.
• The session countdown gets a hairline track that drains as it runs down. This
  reverses an explicit "not a progress bar" line in the UI spec, so the spec is
  updated rather than quietly contradicted. The relay only says when a session
  expires, never how long it was granted, so "full" is whatever remained when we
  first heard.

Also fixes a duplication the new type scale made obvious: with the pairing QR
gone, the connection state was rendered both as the heading and as a status row
below it, printing the same four words twice. It is one promoted status line now.

Scan values are still sent and displayed raw — the Wi-Fi password the design
mock shows masked is not masked here, because the receiver must show exactly
what it will copy.