fix: select forced theme palettes by a data attribute

[alekswebnet]

Summary

Follow-ups from the review of #58 (keep an explicit LIGHT theme light on a dark OS).

• Forced palettes are selected by a data attribute, not by inline-style text. The theme matched the forced DARK palette with [style*="color-scheme"][style*="dark"] and, since Respect explicit light theme #58, skipped the system dark rules with [style*="color-scheme: light"]. Both are substring tests on the whole style attribute, so they depend on how the browser serializes the inline style and misfire on values such as color-scheme: light dark or an unrelated --x: lightgray. applyViewerTheme() now also sets html[data-color-scheme="light|dark"] (removed for AUTOMATIC), and all 19 guards in the theme match that attribute exactly. The inline color-scheme is still written, since PDF.js and the browser UI use it.
• The forced palette is applied before PDF.js initializes, so the first paint already has it. Previously applyViewerTheme() only ran after initializedPromise.
• LIGHT on a dark OS no longer gets PDF.js's dark hover and disabled styles. viewer.css switches --hover-filter and --button-disabled-opacity on the system scheme alone. The theme now sets both in its light base and its dark blocks, so they follow the chosen theme.
• Parity: the neutral forced-dark .sidebar block gains the --sidebar-box-shadow its @media twin already had.
• Comments explain the :where() / &:root pattern (no added specificity, so consumer overrides at :root level still win) and cross-reference the second palette.
• README: the "Viewer CSS theme" section now says LIGHT and DARK override the system color scheme, and AUTOMATIC (the default) follows it.

Test

The theme test from #58 now also checks data-color-scheme and --hover-filter for every mode, drops a repeated loop iteration, and filters media rules by conditionText instead of a cross-realm instanceof. The dark-block count assertion is kept: it is the only assertion that touches the warm palette's @media block, since the neutral palette hides its colors.

Full suite 21/21, pnpm build succeeds, both attribute guards are present in dist/paper-and-ink.css.

Not included

Driving the theme from color-scheme with light-dark() would remove the duplicated dark palettes (a @media block plus a forced block per palette) and the guards altogether. That rewrites the theme file, so it belongs in its own PR.

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