feat: F4.8c the documentation mode's image export, as Java's

ExportRun measures Java's print panel (GUIContainer.prepareExportPanel) and
image on four projects and an empty one: 10 to spare around, a light grey
footer 20 below the lowest element, 20 below it, a transparent background,
the documentation painted as in the design. The sidebar gains Java's
"Exportieren": the surface painted with Canvas 2D (an SVG with a
foreignObject taints the canvas in WebKit) and saved under the project's
name; Java's message when saving fails. Not adopted: Java's empty image of
an empty project (its footer overflows). The editor takes a projectName;
the capture export shares the download helper.

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

packages/editor/src/i18n/de.json

@@ -104,9 +104,12 @@
   "dnsServer.tabNS": "Nameserver (NS)",
   "dnsServer.tables": "Einträge",
   "docu.color": "Farbe",
+  "docu.export": "Exportieren",
+  "docu.exportHint": "Als Bild exportieren",
   "docu.fontSize": "Schriftgröße",
   "docu.fontStyle": "Schriftstil",
   "docu.rectangle": "Gliederungsfläche",
+  "docu.saveFailed": "Fehler beim Speichern aufgetreten!",
   "docu.sidebar": "Dokumentation",
   "docu.sizeLarge": "Groß",
   "docu.sizeNormal": "Normal",