fix(css): improve light-mode comment and callout contrast

[hakula139]

Summary

• Use the shared theme tokens for Twikoo placeholders, metadata, footer, sorting, actions, buttons, and input focus. Validation messages and invalid-input borders use the existing danger palette. Reaction icons inherit their parent action color, including hover states, so their SVG fills follow the same tokens. Twikoo inherits the main text color, but its injected Element UI styles keep pale placeholders and fixed blue actions in light mode. The overrides exceed upstream specificity so they work when Twikoo injects its styles after the theme stylesheet.
• Darken light-mode callout foregrounds that were difficult to read on pale panels. Keep the matching print reset values. The print stylesheet deliberately repeats light literals with !important so printing while dark mode is active still resolves the light palette, following the existing token contract.
• Use the existing semantic palette for Twikoo's blue, green, red, and yellow badges. A shared badge color drives the foreground, border, and opaque theme-tinted background, keeping small labels legible against image-backed panels.
• Group comment styles by responsibility, keep base rules before state overrides, and remove redundant comments across the theme stylesheets and simplify print selectors. Rebuild the committed stylesheet.

Verification

• pnpm build, pnpm format, pnpm lint, pnpm spellcheck, nix flake check, and git diff --check passed.
• Chrome DevTools inspected live Twikoo DOM and injected CSS on the LR2 tools article. A compiled CSS overlay verified light → dark → light changes, input focus, and enabled / disabled send colors. Checks loaded the patch before the injected styles to verify cascade precedence. Reaction icon spans, SVGs, and paths matched parent colors in light and dark modes, including actual hover states for outline and solid icons.
• Audited shared theme colors, including image-backed home cards, the pinned indicator, search tokens, and embeds. Confirmed the additional callout foreground issue and preserved intentional image-backed white text.
• The consuming site built and indexed 75 pages. Headless Chromium at 1440px and 390px verified light → dark → light, visible reaction icons and hover, placeholders, input focus, enabled / disabled send, local preview, emotes, and search. All four supported badge colors exceed 5:1 contrast against their surfaces in both themes. Focused invalid email inputs retain the danger border, and error messages resolve the same semantic color. Printing from either theme resolves the light palette.
• The comment-only follow-up rebuilt theme and site stylesheets byte-for-byte identically. Local theme formatting, lint, spelling, and Nix checks passed. Theme CI and all three dispatched site CI jobs passed on the final heads. After integrating the existing theme/site main changes to resolve the submodule conflict, automatic site CI and preview deployment passed. Direct hosted-preview desktop/mobile checks also passed, including the badge on the LR2 article. Other browser engines and incompatible Nix systems were not tested locally. No production deployment or merge is included.