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/advanced-topics/index.md

@@ -0,0 +1,10 @@
+# Advanced Topics
+
+This section covers advanced topics in system design and architecture.
+
+## Topics
+
+- Distributed Systems
+- Consensus Algorithms
+- Database Internals
+- Network Protocols