Align the platform look and feel with the devplace.net terminal design

Retheme every token in variables.css to the terminal design language:
near-black surfaces (#0a0a0f/#111118/#1a1a24), terminal green accent
(#22c55e) with dark on-accent text, gray text ramp, #222233 borders,
sharp 4-8px radii, neutral depth shadows, and JetBrains Mono as the
primary UI font (vendored latin woff2, no CDN).

Follow-through outside the token file: white-on-accent buttons/tabs now
use the new --on-accent token for contrast, three var() fallback
violations shipping the old palette are gone, the dead --bg-hover token
is replaced with --bg-card-hover, avatar fallback SVG, statistics chart
palette, chat light-theme overrides, offline page, PWA manifest,
theme-color meta, OG image + app icons regenerated in the new palette,
and the /docs/styles-colors style guide now documents the new values.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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