docs: update all docs with dev server, live reload, Juice inlining

- README.md: restructure Preview section into Static + Dev modes
  with feature comparison table; mention juice inlining and live reload
- CONTRIBUTING.md: update Development Setup with Go/Node.js
  requirements, static vs dev preview workflows, Juice reference
- AGENTS.md: add dev command to subcommand list, document
  tools/server/ directory, update Build Tool section
- docs/CONTRIBUTING.*.md (5 languages): add dev server section,
  Node.js requirement, live reload description
- docs/README.*.md (5 languages): add dev mode to Preview section
- docs/images/README.md: update capture instructions for dev server

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/README.ru.md

@@ -36,11 +36,18 @@ systemctl restart gitea
 
 ## Предпросмотр
 
+**Статический режим:**
 ```bash
 cd tools && go run . preview all
 ```
 Затем откройте `preview/index.html`.
 
+**Dev-сервер (live reload + Juice CSS inline):**
+```bash
+cd tools && go run . dev     # требуется Node.js
+# → http://localhost:3456
+```
+
 ## Совместимость
 
 - **Gitea 1.21+**, 100% совместимость, только встроенные функции