feat(ui): put the web app on the documentation site's palette

[vishr]

The web app ran on Mantine's stock theme — teal primary, Inter named in the font stack but never loaded, and no dark mode at all. The documentation site has a palette with reasoning behind it (Ayu Dark surfaces, a violet accent, IBM Plex split by role) and a comment claiming the app already shared it. It did not: not a hex, not a typeface, not a variable name crossed between them.

The collision

healthColor() returned "teal" for a healthy service while primaryColor: "teal" painted every button, link and badge. One hue meant both "healthy" and "clickable" on the same screen. The site picked violet precisely to avoid this — violet is the one hue in the palette that carries no status meaning.

What this does

• ui/tokens.ts — new. The Ayu ramp, the violet accent, the three health hues (#7fd962 / #ffb454 / #f26d78, the site's own), a fourth info hue, chart chrome, and a categorical series palette drawn from the brand mark's ribbons. Values mirror site/src/styles/fanout.css; the two remain separate sources kept in step by hand.
• ui/theme.ts — binds those to Mantine under semantic names: brand, ok, warn, bad, info. A component asks for the accent or a health state and gets whatever the palette holds, so re-hueing is a one-file change. primaryShade: {light: 7, dark: 5} picks the accent each scheme already uses on the site; autoContrast handles the two-shade fill; Mantine's dark ramp is replaced by Ayu, which lands the app on the documentation's exact ground.
• Dark mode, for the first time: defaultColorScheme="auto", a header toggle, and the scheme resolved by an inline script in index.html's head so the first paint lands on the right ground. Paired theme-color metas replace the stray #f6f5f1.
• The typeface ships rather than being named: IBM Plex Sans + Mono, eight weights in the host app, four inlined into each single-file embedded view. assetsInlineLimit had to be raised or the build would have emitted loose .woff2 files it never copies into internal/mcp/apps.
• Charts take resolved values from the same ramps (chartTheme / statusHex / seriesColor) instead of six hardcoded green-greys and a Mantine hue set that included the health colors.

Bugs this surfaced

All three were invisible while the app was light-only:

• The sign-in card hardcoded rgba(255, 255, 255, 0.9), so the auth page would have stayed white under a dark app — with no toggle on that screen to escape.
• Dashboard metric tiles used bg="gray.0", the same color in both schemes; the four health tiles rendered white-on-white.
• mcp-app-frame.tsx passed theme: "light" as a literal to the app bridge, which would have left every embedded MCP view light inside a dark host.

Verification

just check passes end to end. Exercised against a real binary with 3,746 seeded spans in both schemes, toggling each direction, plus the signed-out auth page in dark.

Two things reviewers should know: the embedded MCP views are type-checked but not visually exercised (chat needs an AI provider key), and running just check on Go 1.27 fails TestAppendJSONString_MatchesEncodingJSON and regenerates THIRD_PARTY_NOTICES with one extra component — go.mod pins 1.26.5 and that is what CI and this branch used.

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