Add design-system foundation (tokens, theming, accent, color scheme)

[b006n97]

Introduces the platform layer the rest of the UI overhaul sits on, with no feature redesigns yet:

• theme/tokens.ts: single source of truth for accent ramps (red/blue/green), per-scheme neutrals, medal/placement colors, and timing.
• theme/index.ts: createAppTheme(accent) builds a colorSchemes (light/dark) theme, exposing accent + neutrals as CSS vars (--mui-palette-*).
• theme/AccentContext.tsx: runtime accent switch (localStorage-persisted), orthogonal to MUI's color-scheme mode.
• theme/utils.ts: additive sx presets (gradientTitle, eyebrowLabel) plus restyled existing helpers; export names/types unchanged so current components keep compiling.
• index.tsx: InitColorSchemeScript to avoid a flash of the wrong theme.
• index.html: load the Manrope font used by the theme typography.
• App.tsx: swap ThemeProvider(createAppTheme()) for only; routing is left untouched here and migrated in later PRs.
• DESIGN.md: design-system reference. Fixed the tokens.ts docstring drift (was "two accent variants"; there are three: red/blue/green).

Typechecks independently (tsc --noEmit clean) with the still-un-migrated components consuming the new theme.