feat: optional device images in rack elevations, HTML preview in new tab

- New export option "Gerätebilder anzeigen": the SVG is rendered with
  include_images and the /media/ image links are replaced by embedded
  data URIs, since Word, WeasyPrint and SVG-as-image cannot load
  login-protected files. The Pillow PNG renderer draws the front/rear
  images too, with the device name on a dark label.
- Custom rack_svg_renderer adapters receive images=..., older adapters
  without that parameter keep working.
- The HTML preview now opens in a new browser tab.

Bump version to 0.3.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "netbox-customer-export"
-version = "0.2.0"
+version = "0.3.0"
 description = "NetBox-Plugin: Kundendokumentation (PDF/Word) für Standorte, Lokationen, Mandanten und Mandantengruppen"
 readme = "README.md"
 requires-python = ">=3.10"