dsh-ai-nav: Add an AI navigation wall to the DSH sidebar

The DSH plugin maintained by naturalth2012 provides 95 verified AI site entries across eight categories in the sidebar. It includes a search box, site-specific search, scenario-based recommendations, pinned sites and recent visits, and bookmark import. The plugin exposes six tools to the host—ai_nav_search, ai_nav_recommend, ai_nav_browse, ai_nav_sites, ai_nav_import, and ai_nav_open—ensuring that URLs retrieved by the model are real and clickable. The directory is manually maintained according to two rules: every URL is verified to be accessible, and site-specific search templates are provided only when the site supports query parameters. Icons are served via a same-origin proxy, supporting Byte-Range requests and text fallback. Custom sites and pinned items are stored in ai-nav/data.json, with all links filtered through safeUrl(). It provides node scripts/selfcheck.mjs for offline self-checking, covering 138 pure function tests.