feat(*): theme system

[nekomeowww]

Summary

• Adds docs theme runtime selection so the default VitePress docs site and theme-specific docs builds can share content while loading different theme entrypoints.
• Splits integration client CSS into shared/common styles plus theme-specific VitePress, VoidZero, and Proj AIRI style layers.
• Adds NolebasePluginPreset({ theme }) support so plugins can register app-level behavior while choosing theme-specific layout slot wiring.
• Adds the VoidZero enhanced-readabilities header adapter from the package export instead of a docs-local workaround.

Validation

• pnpm typecheck
• pnpm lint:fix
• Commit hook: pnpm -r --filter=./packages/* run stub
• Commit hook: pnpm -r --parallel run typecheck
• Commit hook: staged eslint --fix
• pnpm -F @nolebase/integrations-docs docs:build:theme:voidzero
• Browser validation against http://127.0.0.1:41732/themes/voidzero/pages/en/integrations/

Notes

VoidZero currently lacks a right-side header slot equivalent to VitePress nav-bar-content-after, so the enhanced-readabilities VoidZero adapter uses a documented Teleport workaround until VoidZero exposes an appropriate slot.