feat: Sharp style, Tailwind+shadcn, dark navbar, nav restructure, Go/monorepo guidelines

Styling:
- Tailwind CSS v4 (preflight:false) integrated via PostCSS plugin
- shadcn/ui New York style with --radius:0 (sharp corners everywhere)
- Zero border-radius on ALL elements: buttons, code blocks, cards, admonitions, badges
- Navbar FORCE dark (#0a0a0a) in both light/dark modes via !important
- ThemeSynchronizer: bridges Docusaurus data-theme → shadcn .dark class
- Active sidebar item gets left border accent instead of rounded bg

React UI components (src/components/ui/index.tsx):
- Badge (6 variants: wip, stable, planned, success, warning, outline)
- Card + CardHeader + CardTitle + CardContent
- Callout (note/info/success/warning/danger) - sharp, left-border style
- PropertiesTable - monospace typed API reference tables
- StatusIndicator - operational/degraded/outage/maintenance

Navigation restructure:
- Introduction → Platform → Products → Engineering → Runbooks
- Old 'applications/' removed, moved to products/stock-market-pro/
- New Platform section: infrastructure overview + CI/CD pipeline docs
- New Engineering section: monorepo structure, Go stack, conventions
- New Runbooks section: incident severity, common procedures

Implementation guidelines (engineering/guidelines.md):
- Monorepo layout with go.work workspace
- Go as primary language (why + standard library table)
- Python as sidecar for data/ML workloads
- Code conventions: gofmt, error wrapping, logging, testing
- Git conventions: branch naming, conventional commits
- Makefile targets per app
- How to add a new application (step-by-step)

docs/runbooks/index.md

@@ -0,0 +1,61 @@
+---
+sidebar_position: 1
+---
+
+# Runbooks
+
+Operational procedures for SKIC Playground services.
+
+:::note
+Runbooks are living documents. Update them when procedures change or new issues are discovered.
+:::
+
+## Incident Severity
+
+| Level | Definition | Response Time |
+|---|---|---|
+| P0 | Platform down / all services unreachable | Immediate |
+| P1 | Single critical service down | < 15 min |
+| P2 | Degraded performance / non-critical service down | < 1 hour |
+| P3 | Minor issue / cosmetic | Next working session |
+
+## Common Procedures
+
+### Restart a container
+
+```bash
+# SSH to TrueNAS
+ssh root@lego-cloud.eu
+
+# List running containers
+docker ps
+
+# Restart
+docker restart <container-name>
+
+# Check logs
+docker logs -f --tail 100 <container-name>
+```
+
+### Check Gitea Actions runner status
+
+```bash
+# On the runner host
+systemctl status gitea-runner
+
+# Restart if needed
+systemctl restart gitea-runner
+```
+
+### Force re-deploy from latest image
+
+```bash
+docker pull <registry>/<image>:<tag>
+docker stop <container>
+docker rm <container>
+docker run -d --name <container> ... <image>:<tag>
+```
+
+## Application-specific runbooks
+
+Individual application runbooks will be added here as applications go into production.