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/static/fonts/LICENSE.txt

@@ -0,0 +1,7 @@
+Overpass and Overpass Mono are licensed under the SIL Open Font License,
+Version 1.1: https://openfontlicense.org/
+
+Fetched from Google Fonts (fonts.google.com/specimen/Overpass,
+fonts.google.com/specimen/Overpass+Mono) and self-hosted here rather than
+loaded from a CDN, so the app has no runtime dependency on Google's font
+service.