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

sidebars.ts

@@ -2,38 +2,49 @@ import type {SidebarsConfig} from '@docusaurus/plugin-content-docs';
 
 const sidebars: SidebarsConfig = {
   docsSidebar: [
-    'intro/index',
+    {
+      type: 'doc',
+      id: 'intro/index',
+      label: '← Overview',
+    },
     {
       type: 'category',
       label: 'Applications',
+      collapsed: false,
       items: [
         {
           type: 'category',
-          label: 'Stock Market Professional',
-          link: {
-            type: 'doc',
-            id: 'applications/stock-market-professional/index',
-          },
+          label: '📈 Stock Market Pro',
+          link: { type: 'doc', id: 'applications/stock-market-professional/index' },
           items: [
-            'applications/stock-market-professional/overview/architecture',
-            'applications/stock-market-professional/data-monitoring/index',
-            'applications/stock-market-professional/technical-analysis/index',
-            'applications/stock-market-professional/signal-generation/index',
+            {
+              type: 'category',
+              label: 'C4 Architecture',
+              collapsed: false,
+              items: [
+                'applications/stock-market-professional/c4/context',
+                'applications/stock-market-professional/c4/containers',
+                'applications/stock-market-professional/c4/components',
+              ],
+            },
+            {
+              type: 'category',
+              label: 'Modules',
+              items: [
+                'applications/stock-market-professional/data-monitoring/index',
+                'applications/stock-market-professional/technical-analysis/index',
+                'applications/stock-market-professional/signal-generation/index',
+              ],
+            },
           ],
         },
       ],
     },
     {
       type: 'category',
-      label: 'Architecture',
+      label: 'Platform',
       items: [
         'architecture/overview',
-      ],
-    },
-    {
-      type: 'category',
-      label: 'Guides',
-      items: [
         'guides/getting-started',
       ],
     },