feat: version display + issues board

Version: CalVer + git hash shown in top-right corner of main page.
Built via Docker ARG BUILD_VERSION at build time. Served via /api-version.

Issues Board: shared notepad window for tracking issues with plugin,
overlord, nav files, macros. Stored in openissues.json on server.
- GET/POST/DELETE /issues endpoints
- Draggable window matching Chat/Radar pattern
- Category tags (plugin, overlord, nav, macro, other) with colors
- Add/resolve issues through the UI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -29,6 +29,10 @@ COPY Dockerfile /Dockerfile
 ## Expose the application port to host
 EXPOSE 8765
 
+## Build version (CalVer + git hash, set via --build-arg)
+ARG BUILD_VERSION=dev
+ENV APP_VERSION=$BUILD_VERSION
+
 ## Default environment variables for application configuration
 ENV DATABASE_URL=postgresql://postgres:password@db:5432/dereth \
     DB_MAX_SIZE_MB=2048 \