[codex] document preferences provider and formatAmount behavior

[Flamki]

What changed

• Added docs/preferences.md covering the preferences model, defaults, STORAGE_KEY, hydration, persistence, safe storage behavior, theme application, system media-query handling, accessibility/contrast implications, and formatAmount branches.
• Linked the new guide from README.md.
• Added JSDoc pointers from PreferencesProvider and usePreferences() to the guide.

Closes #250

Validation

• npm run lint failed on pre-existing issues outside this docs change:
  • src/components/ActionPanel.tsx: parse error at export default ActionPanel
  • src/components/__tests__/Breadcrumbs.test.tsx: missing testing-library/no-node-access rule
  • src/components/toast/toast-provider.test.tsx: unused wrappedDismiss
• npm test -- --runInBand failed because ActionPanel.tsx cannot parse in two suites; 48 suites passed, including src/lib/__tests__/preferences.test.tsx.
• npm run build failed because ActionPanel.tsx cannot parse.
• npm test -- src/lib/__tests__/preferences.test.tsx --runInBand passed all 51 assertions, but Jest exited non-zero due existing obsolete snapshot files.

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