Design a high-fidelity desktop web UI mockup, 16:10, rendered as a flat screenshot -- no browser chrome, no laptop frame, no hands, no desk, no perspective.

Brand: Subnaut. A developer tool. Tone: precise, quiet, engineered. Not playful, no gradients, no glow, no 3D, no stock-photo people, no purple-blue SaaS look.

Palette, used exactly:
  background   #ffffff
  raised panel #fbfbfa
  primary text #0a0a0a
  body text    #46464a
  muted text   #8a8a90
  hairline     #e8e8e6
  accent       #12a37a  (a restrained green -- use sparingly, for one action only)

Type: a clean grotesque sans (Inter/Helvetica feel) for prose. A monospace face for labels, eyebrows, code and buttons -- uppercase, wide letter-spacing, small.

Layout: generous whitespace, a strict 12-column grid, thin 1px hairline rules as the main separator instead of borders or shadows. Left-aligned. Flat.

Mood reference: the Bittensor site, Linear's docs, Stripe's density -- engineering documentation, not a marketing splash.

Page: documentation landing, inside a docs shell.

Three-column: a left sidebar of collapsible navigation (Getting Started, User Guide, Developer Guide, Reference), a wide centre column, a right "On this page" table of contents in muted small type.

Centre: a page title, a one-line description, then a grid of six cards -- each a hairline rectangle with a small monoline icon, a short bold title and one muted line. Cards are flat: no shadow, no fill, hairline border only, generous internal padding.

Below the grid, a two-column list of links with monospace section labels above them.

The sidebar has one active item marked by an accent left-edge rule, not a filled pill.

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If the result is wrong, reply with one of these:
  Use the accent on exactly one element. Everything else is black, grey and white.
  Remove all gradients, shadows, rounded cards and illustrations. Flat, hairline rules only.
  Double the whitespace. Reduce to half the elements.

Dark theme: background #0a0a0a, raised #131313, primary text #f4f4f5, muted #9b9ba3, hairline #26262b, accent #4fc69a.
Mobile: render at 9:16, single column, hamburger in the top bar.
