feat(control-plane): refresh the UI design system

[nicoloboschi]

Refreshes the control plane's design system. The stock shadcn oklch greys are replaced with the Hindsight palette and the shared primitives move onto the design system's density, so the whole app picks up the new look from tokens rather than per-component edits.

Tokens (globals.css)

• Blue-shifted neutral palette in both modes: page #F3F5F9/#080C17, card #FFFFFF/#0F1724, sidebar #FFFFFF/#0A1020, hairline borders (rgba(0,0,0,0.08) / rgba(100,160,255,0.10)).
  In light mode --card was previously the same value as --background, so cards were indistinguishable from the page behind them.
• Adds the --hs-* semantic layer (surface / foreground / border / status / chart) plus its @theme inline mappings, so pages can reach for bg-success, text-chart-world etc. instead of raw hex.
• --tracking-normal 0.025em → 0. Inter reads wrong with positive tracking at body sizes.
• Light --muted-foreground is #525866 — 5.3:1 against the page and 7.1:1 against card white, so body copy clears WCAG AA in both modes.

Fonts

Inter and JetBrains Mono move to next/font/google. The Google Fonts @import they replace loaded weights lazily, which left semibold headings rendering in the system fallback until the weight arrived. Space Grotesk is dropped — it was only reachable through a font-heading utility that no component used.

Primitives

card, button (plus a gradient variant for the primary CTA), input, textarea, select, switch, checkbox, dialog, alert-dialog, popover, dropdown-menu and command move to 13px / h-9 / rounded-[10px], with rounded-[16px] cards and dialogs and blurred overlays.

Layout

• Bank page content is capped by a responsive staircase (1024 / xl:1280 / 2xl:1440, centered). It was previously uncapped, so on a 16" display body text and table rows spanned the full ~1700px window.
• Active tab underlines use the brand gradient rather than flat --primary. The bank-config strategy tabs needed converting from border-b-2 border-primary to an absolutely-positioned bar, since a border can't carry a gradient; the transparent border-b-2 stays so inactive tabs keep their hover underline.
• The sidebar toggles from anywhere on its chrome, not just the collapse button. The keydown handler is scoped to the aside itself so pressing Enter on a focused nav link navigates without also collapsing the rail.
• The three dashboard panels and the entities graph panel were bordered but transparent, which showed page-grey inside a card-looking box under the new palette; they now carry a card surface.

Deliberately preserved

• The h1–h6 weight default. Tailwind preflight resets headings to font-weight: inherit, and not every heading in this app carries an explicit weight utility, so dropping the rule would flatten them.
• The chip tokens (facet-chip), logo keyframes, themed scrollbars and .prose table rules, which a wholesale token rewrite would have dropped.
• Nested bg-muted boxes (code blocks, tool-call rows, chunk lists) stay at rounded-lg. The 16px radius is for card surfaces; promoting nested content boxes to it makes sub-content read as cards.

Review notes

• page.tsx is mostly re-indentation from wrapping the views in the width container — git diff -w shows the actual change.
• No new primitives were added even where the design system defines them (badge, skeleton, tooltip): nothing imports them yet and knip's orphaned-file check is blocking in CI.

Verification

• ./scripts/hooks/lint.sh passes; check-unused.sh reports no blocking findings.
• 101/101 control-plane tests pass.
• npm run build compiles clean (54/54 static pages).
• Exercised manually against a live API in both themes: sidebar collapse/expand, bank config tabs, the configuration form (selects, inputs, textarea, switch, checkbox) and the Add Document dialog.