feat(design-explorer): the drift panel — rulebook says X, tokens say Y

Milestone #251 step 5 (#2262), plus the Settings control that makes it reachable.

The comparison is deliberately thin — set arithmetic over live token values,
which is the one thing the browser knows and the server doesn't. The hard half
(prose to claims) already lives in Python where pytest can assert on it.

Three claim kinds, and the third inverts the test: a `token` claim asks whether a
custom property of that name exists; a `color` claim asks whether any token
resolves to that value; a `prohibited_color` claim FAILS when present.

normalizeColour is the client-side twin of normalize_hex and has one job the
server cannot do: getComputedStyle reports colours as rgb()/rgba() regardless of
how they were authored. So one colour has three spellings in play — #FFFFFF in
the rulebook, #fff in the stylesheet, rgb(255,255,255) from the browser — and a
comparison that misses any of them under-reports silently rather than erroring.

THE PANEL STATES ITS OWN BLIND SPOT, which matters more than it sounds. This
compares the rulebook against TOKENS. A literal hardcoded in a component, where
a token should have been referenced, is invisible to it — the drift isn't in the
tokens at all (#2275: 67 hardcoded whites against a rule forbidding pure white).
Reading those would mean bundling every SFC's source into the app; the check
belongs in CI and is tracked at #2277. A drift report that silently omitted a
whole category would invite the reader to conclude the category is clean, so the
panel says so in the panel rather than in a comment nobody reads.

Findings are ranked violated → missing → ok, and `ok` rows are hidden behind a
toggle. Same principle the auto-inject menu is built on: a short list that gets
read beats a complete one that doesn't.

Settings gains a rulebook picker. "None" is a first-class choice, not an unset
error — most installs have no rulebook describing their design system, and
saving empty DELETES the setting rather than storing a zero. The panel's empty
state points at Settings and Settings points back at the panel, so neither is a
dead end.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/api/design.ts

@@ -0,0 +1,9 @@
+import { apiGet } from "@/api/client";
+import type { ExpectationResponse } from "@/utils/designDrift";
+
+/** Checkable claims from the rulebook this install designated as its design system.
+ *
+ *  `rulebook_id: null` means none has been designated — the normal state for a
+ *  fresh install, not an error. The caller shows an explanatory empty state. */
+export const fetchDesignExpectations = () =>
+  apiGet<ExpectationResponse>("/api/design/expectations");