Feat/theming on docs and lib

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What does this PR do?

Adds a first-class, opt-in theming system shared across the web and React Native renderers, and makes the demo app fully themeable and mobile-responsive.

Library

• New theme prop on MdmaDocument (both renderers): "light" | "dark" | "auto" (follows the OS) or a full MdmaTheme token object. Omit it for the unchanged default light look — fully backward compatible.
• Web renderer: styles.css is now driven by --mdma-* CSS variables; built-in palettes apply via data-theme, custom themes via inline variables (and are overridable directly in CSS). Exposes MdmaThemeProvider, useMdmaTheme, lightTheme, darkTheme, and helpers. A themeless MdmaDocument inherits an ancestor MdmaThemeProvider's theme.
• Web renderer polish: a custom dark theme now picks the correct light/dark base (by background luminance) so internal colors (headings, code) stay readable; button hover + input focus ring derive from the accent via color-mix.
• React Native renderer: theme="auto" via useColorScheme(); the built-in lightTheme/darkTheme palettes are unified with the web renderer's so MDMA content looks identical across platforms; fixed TableRenderer/ChartRenderer whose bodies had no themed background (rows showed through on dark).
• Both renderers share the same MdmaTheme shape, so a theme object is portable.

Demo app

• Header light/dark/auto toggle that themes the whole app (chrome, nav bar, docs side nav, and all live examples).
• New Theming docs page with a live theme editor (5 color pickers — accent/background/text/muted/border — re-rendering components instantly).
• Docs restructure: Renderers (React, React Native) and Integrations nav groups; generalized the Components catalog; removed the renderer package from the Packages list.
• Mobile responsive across every route, with a hamburger nav drawer + breadcrumb trail on mobile, and thinner theme-aware scrollbars.

Docs: new docs/guides/theming.md; README + RN package README updated.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation

Packages Affected

• @mobile-reality/mdma-renderer-react
• @mobile-reality/mdma-renderer-react-native
• Demo app (@mobile-reality/mdma-demo) + docs

Checklist

• I have read the CONTRIBUTING guide.
• My code follows the existing code style (pnpm format and pnpm lint pass).
• I have added or updated tests that cover my changes.
• All tests pass (pnpm test).
• Type-checking passes (pnpm typecheck).
• I have added a changeset (pnpm changeset) if this change affects published packages.
• New or changed MDMA schemas are backwards-compatible (or marked as breaking). — N/A, no schema changes
• Sensitive fields are marked with sensitive: true where appropriate. — N/A

How to Test

1. pnpm install && pnpm demo, open the demo.
2. Use the ☀️ / 🌙 / 🖥️ toggle in the header — the whole app (chrome + all live examples) switches theme.
3. Go to Docs → Theming and use the live editor: change the 5 color pickers and watch the components (and their hover/focus states) update; toggle light/dark to reset.
4. Visit Docs → React Native — the RN preview (via react-native-web) uses the same unified palette; try dark mode and the Adoption table example (body themes correctly).
5. Custom theme in code: <MdmaDocument ast={ast} store={store} theme={myTheme} /> (spread lightTheme/darkTheme and override tokens); or set --mdma-* CSS variables directly.
6. Resize to a phone width (~390px): docs side nav becomes a hamburger drawer + breadcrumb; all routes stack with no horizontal scroll.
7. pnpm typecheck && pnpm test && pnpm lint — all green.