feat: Modern styling, draw.io integration, and C4 architecture docs

Styling:
- Dark-first design (Vercel/Linear inspired) with Geist font
- Custom CSS with full --ifm- token overrides
- Glassmorphic navbar, refined sidebar, modern tables
- Gradient hero with animated glow, app cards, C4 section
- Custom badge, tag, and callout components
- Announcement bar, TOC config, dark scrollbar

draw.io:
- Added docusaurus-plugin-drawio + raw-loader
- SMP full pipeline diagram (smp-pipeline.drawio)
- Embedded in architecture overview

Mermaid:
- Added @docusaurus/theme-mermaid
- C4Context, C4Container, C4Component diagrams
- Sequence and graph diagrams in architecture

C4 Architecture (Stock Market Professional):
- L1 Context: system in the world, actors, external systems
- L2 Containers: Data Ingestor, Analysis Engine, Signal Generator, TimescaleDB, Notifier
- L3 Components: deep component breakdown with mermaid C4Component diagrams

Other:
- Sidebar restructured with C4 section and Modules section
- Homepage rebuilt as modern dark landing page

package.json

@@ -18,9 +18,12 @@
     "@docusaurus/core": "3.10.2",
     "@docusaurus/faster": "3.10.2",
     "@docusaurus/preset-classic": "3.10.2",
+    "@docusaurus/theme-mermaid": "^3.10.2",
     "@mdx-js/react": "^3.0.0",
     "clsx": "^2.0.0",
+    "docusaurus-plugin-drawio": "^0.4.0",
     "prism-react-renderer": "^2.3.0",
+    "raw-loader": "^4.0.2",
     "react": "^19.0.0",
     "react-dom": "^19.0.0"
   },