fix: dark mode visibility across all components

[drewstone]

Summary

Users reported that nothing is visible on the docs site in dark mode. This PR adds missing dark: Tailwind variants to all components that had hardcoded light-mode-only styles.

Changes

Components fixed:

• HelpDiscordBtn.tsx — Added dark:bg-gray-800 dark:text-gray-200 dark:border-gray-600 dark:hover:bg-gray-700
• DiscordButton.tsx — Same dark mode overrides
• DiscordBanner.tsx — Changed text-gray-400 to include dark:text-gray-300 for better contrast
• ExpandableImage.tsx — Added dark backgrounds to zoom/reset buttons in the image modal
• TokenContracts.tsx — Added dark text variants to:
  • Token symbol text (dark:text-gray-400)
  • Search icon (dark:text-gray-400)
  • Search input (dark:text-white dark:placeholder-gray-400)
  • Chain select dropdown (dark:text-white)

What was already fine

• linkBlue usages already had dark:text-blue-200 or dark:text-white overrides
• tangleLogoTextColor in Footer already had dark variants
• Shiki code block line numbers already use appropriate colors for dark mode

No light mode appearance changes.

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