Retarget the theme to the devplace main-branch design (indigo + rust)

The reference design is the devplace Next.js main branch, not the
terminal branch the previous commit tracked. Swap the token values to
that design language: deep indigo page (#271b5b) over near-black indigo
cards (#13112a/#1a1736), #11101f hairlines, purple-tinted text ramp
(#e9e4ff/#c8c3e5/#9b93c9), burnt-orange primary accent (#b73f1e) with
a raspberry hover (#af3050) and a rust-to-plum brand gradient, white
on-accent text, soft rounded radii (8/12/16px), the reference's diffuse
soft/medium/strong shadows, a warm accent glow, and Trebuchet MS as the
UI font (system font - the vendored JetBrains Mono files are removed
along with their base.html link).

Follow-through updated to match: theme-color meta + PWA manifest
(#271b5b), offline page, avatar fallback SVG, statistics chart palette
(brand ramp), chat light-theme overrides, OG image + app icons
regenerated in the new palette, and the /docs/styles-colors guide.

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

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