Typed design-token system

[ThomasHartDev]

adds a small typed design-token tree in src/tokens that acts as the single origin for the app's design values. instead of hand-writing the same CSS variables twice, the token module generates the :root custom properties and the root layout injects them at render time, so there's one source and nothing to drift. the token() helper hands back a var() reference and is generic over the tree, so a wrong name is a compile error rather than a silent typo. also threw in a /tokens route that renders the swatches straight from the tree (stays a server component, no client JS) and wired the home page link to it. tests cover the kebab-casing, the var generation, empty groups, and the round-trip back into a :root rule anyway.

[ThomasHartDev]

walked through this again before merging. src/tokens/tokens.ts holds the values as one as const tree and exports the group/key types off it. index.ts is the only thing that knows how a token becomes a CSS name: varName() does the kebab-casing once, cssVarName/token wrap it with the generic key constraint so callers can't pass a name that isn't in the tree, and tokensToCssVariables/cssVariablesToRootRule walk the tree into a :root block. layout.tsx calls those at module scope and drops the result into a <style>, which is why globals.css lost its hardcoded token block. next build confirms / and /tokens both prerender static with no client JS, so the token route is a pure server component. only thing i left for later is light/dark theming, which is a second :root under a media query and belongs in its own PR.