feat: follow the reader's light or dark setting

[SurefireStudios]

Closes #14.

Both surfaces share this stylesheet, so the light theme is a prefers-color-scheme: light block that redefines the tokens and nothing else. Every rule below stays written once and both themes get it, and the dashboard layer still defines no token of its own.

One correction to the issue

The issue says all colours are already tokens. Nineteen were not, and they were exactly the ones that break:

• Raised surfaces were literal white overlays, rgba(255, 255, 255, a), in eighteen places: nav hover, table stripes, chips, the button's lit top edge. White on a white page is invisible, so the whole depth model would have flattened.
• The sticky topbar carried the dark page colour, rgba(8, 9, 12, 0.82), so it would have floated a dark band over a light page while scrolling.

Rather than tokenise eighteen values separately, the overlays now multiply against a single --tint that inverts with the theme, preserving every existing alpha unchanged. The topbar gets a --scrim. That is two new tokens instead of eighteen, and it keeps the file's rule that depth is expressed one way.

The verdict colours are not the dark ones lightened

--pass, --fail and --review are chosen for near-black and do not hold contrast on white. They are darkened for the light theme instead of reused. The file's stated principle is that a verdict is read as a word and a glyph first, so colour is never the only signal, but it still has to carry, and the claim/reality contrast is still the one dramatic moment on the page.

Constraints respected

• No new rule and no inline style, so the Content-Security-Policy is untouched.
• The dashboard layer is not modified, so ships the product stylesheet verbatim as its base and defines no token or base component of its own both still hold.

Verification

• apps/product and apps/dashboard tests: 79 passing, including the two that guard the shared-stylesheet arrangement and the one that fails the build on an inline style.
• pnpm typecheck passes.
• Checked in a browser in both schemes: light renders correctly across the topbar, hero, buttons, cards, the claim/reality panels and the verdict badges, and dark is unchanged.

Note

Low Risk

Overview
This pull request implements automatic light/dark theme switching based on the user's system preference via prefers-color-scheme: light media query in apps/product/src/client/styles.css. The change corrects a prior assumption that all colors were already tokenized by introducing tokens for nineteen previously-hardcoded values, including raised-surface overlays that used literal rgba(255, 255, 255, a) values. The structure keeps every non-tokenized rule written once and shared between themes, with the light theme only redefining token values while the dashboard layer continues to avoid defining tokens of its own. Overall, this enables both light and dark surfaces to share the same stylesheet while honoring the reader's OS-level theme setting.

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