feat(icon): Phosphor glyph card — hero + real-UI-size strip (T-313)

The icon card renders each glyph at a hero size plus a continuous sample
strip (10–48px) so legibility is judged at the sizes the app uses, with the
entry's optional label + description and a per-entry or card-level color.
Wired end to end: icon.show publishes on the `icon` bus, the Claude
extension injects an IconMessage, and conversation_view paints the card
(display-only, D-78). en+nl catalogs.

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

assets/i18n/en_us/builtin.claude.json

@@ -47,6 +47,7 @@
   "conversation.label.thinking": { "translation": "thinking" },
   "conversation.label.agentThinking": { "translation": "agent thinking" },
   "conversation.label.image": { "translation": "image" },
+  "conversation.label.icon": { "translation": "icons" },
   "conversation.label.drawing": { "translation": "drawing" },
   "conversation.draw.viewSource": { "translation": "view d2 source" },
   "conversation.label.agentRun": { "translation": "agent run" },

assets/i18n/nl_nl/builtin.claude.json

@@ -47,6 +47,7 @@
   "conversation.label.thinking": { "translation": "nadenken" },
   "conversation.label.agentThinking": { "translation": "agent denkt na" },
   "conversation.label.image": { "translation": "afbeelding" },
+  "conversation.label.icon": { "translation": "iconen" },
   "conversation.label.drawing": { "translation": "tekening" },
   "conversation.draw.viewSource": { "translation": "d2-bron tonen" },
   "conversation.label.agentRun": { "translation": "agent-uitvoering" },