feat: add interactive file path cards in chat UI

Render filepath code blocks as rich, clickable cards with three variants:
knowledge files (navigate to editor), audio files (inline play/pause),
and system files (open externally). Adds shell:openPath and
shell:readFileBase64 IPC channels, FileCardProvider context, and
Streamdown pre override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/x/packages/shared/src/ipc.ts

@@ -386,6 +386,15 @@ const ipcSchemas = {
       success: z.literal(true),
     }),
   },
+  // Shell integration channels
+  'shell:openPath': {
+    req: z.object({ path: z.string() }),
+    res: z.object({ error: z.string().optional() }),
+  },
+  'shell:readFileBase64': {
+    req: z.object({ path: z.string() }),
+    res: z.object({ data: z.string(), mimeType: z.string(), size: z.number() }),
+  },
 } as const;
 
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