feat: miller column browser, offline cache, search, breadcrumbs

- Dynamic miller column sizing with flexbox layout
- Root column (160px), middle slices (48px), active column (flex)
- Auto-scroll browser to rightmost column on navigation
- Offline indicator UI and IndexedDB cache integration
- /api/documents endpoint for client-side document caching
- Breadcrumb navigation in document content area
- FTS5 search with SQLite virtual table
- Client-side Mermaid and KaTeX rendering via CDN
- Deep sample content (advanced-topics/raft-deep-dive)
- Border removal (only search button keeps radius)
- Full viewport layout with proper borders between sidebar/content

content/index.md

@@ -0,0 +1,16 @@
+# Welcome to MD Hub Secure
+
+This is your documentation hub. Browse documents using the sidebar, or explore the features below.
+
+## Getting Started
+
+- [[getting-started]] — Learn the basics
+- [[architecture-overview]] — Understand the system design
+- [[project-plan]] — See what's on the roadmap
+
+## Features
+
+- Markdown rendering with wiki-link support
+- Tag extraction and filtering
+- Content-addressed attachment storage
+- Real-time document syncing