feat(draw): fold the d2 source into a "view source" disclosure (T-494)

The rendered diagram leads; the d2 source carries through the draw bus and
folds into a collapsed ClideCollapserCard beneath the card (D-78 display-
only). en+nl catalogs. Also switches the d2 test to a null-aware element.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

assets/i18n/en_us/builtin.claude.json

@@ -48,6 +48,7 @@
   "conversation.label.agentThinking": { "translation": "agent thinking" },
   "conversation.label.image": { "translation": "image" },
   "conversation.label.drawing": { "translation": "drawing" },
+  "conversation.draw.viewSource": { "translation": "view d2 source" },
   "conversation.label.agentRun": { "translation": "agent run" },
   "conversation.label.workflow": { "translation": "workflow" },
   "conversation.label.error": { "translation": "error" },

assets/i18n/nl_nl/builtin.claude.json

@@ -48,6 +48,7 @@
   "conversation.label.agentThinking": { "translation": "agent denkt na" },
   "conversation.label.image": { "translation": "afbeelding" },
   "conversation.label.drawing": { "translation": "tekening" },
+  "conversation.draw.viewSource": { "translation": "d2-bron tonen" },
   "conversation.label.agentRun": { "translation": "agent-uitvoering" },
   "conversation.label.workflow": { "translation": "workflow" },
   "conversation.label.error": { "translation": "fout" },