Build the Signal design system: tokens, fonts, theme, density

Self-hosted variable fonts (Overpass/Overpass Mono, OFL-licensed) --
no CDN dependency for the app to render correctly. Dark is the literal
default in tokens.css (:root defines it directly; light is the
override via both prefers-color-scheme and an explicit data-theme),
not a retrofit. Severity tokens collapse OTel's seven severities to
five visual tiers (severity.ts); their translucent -bg variants and
light-mode warn's base color are already tuned for WCAG AA contrast
against an opaque surface, not just the plain page background --
verified with real axe-core runs during the accessibility pass, see
docs/design-system.md.

theme.svelte.ts/density.svelte.ts persist to localStorage and expose
getter/setter functions wrapping module-level $state (Svelte 5's
shared-state-module pattern -- a directly exported $state doesn't
preserve reactivity across modules). app.html's inline script applies
both before first paint to avoid a flash of the wrong theme/density.

web/src/lib/theme.svelte.ts

@@ -0,0 +1,49 @@
+// Theme is dark/light/"system" -- but unlike most apps, an unset
+// preference means dark, not system. That's the whole point of "real
+// dark mode as the default, not an afterthought": a first-time visitor
+// on a light-OS machine still lands in dark, matching the actual usage
+// pattern this product is designed for (long sessions, often at night).
+// "system" is available for anyone who wants their OS setting to win
+// instead, but it's a deliberate opt-in, not the fallback.
+//
+// The synchronous inline script in app.html applies the same stored
+// value before first paint -- this module is what UI controls read/write
+// after hydration; the two must stay in sync on the storage key and
+// values, not just in spirit.
+
+export type Theme = 'dark' | 'light' | 'system';
+
+const STORAGE_KEY = 'sentry.theme';
+
+function readStored(): Theme {
+	if (typeof localStorage === 'undefined') return 'dark';
+	const v = localStorage.getItem(STORAGE_KEY);
+	return v === 'light' || v === 'system' ? v : 'dark';
+}
+
+function apply(t: Theme) {
+	if (typeof document === 'undefined') return;
+	if (t === 'system') {
+		document.documentElement.removeAttribute('data-theme');
+	} else {
+		document.documentElement.setAttribute('data-theme', t);
+	}
+}
+
+const initial = readStored();
+let theme = $state<Theme>(initial);
+apply(initial);
+
+export function getTheme(): Theme {
+	return theme;
+}
+
+export function setTheme(t: Theme) {
+	theme = t;
+	apply(t);
+	try {
+		localStorage.setItem(STORAGE_KEY, t);
+	} catch {
+		// storage unavailable -- the choice just won't persist across reloads
+	}
+}