feat: implement Phase 8 - Process Stdin

Features added:
- Created stdin API route for sending input to process stdin
- Added useSendProcessStdin React Query hook
- Created StdinInput modal component with:
  - Multi-line textarea for input
  - Ctrl+Enter keyboard shortcut
  - Optional newline append
  - Character count display
  - Info banner explaining stdin functionality
- Added stdin button to ProcessCard (Terminal icon)
- Button only enabled when process is running (state === 20)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/hooks/useSupervisor.ts

@@ -645,3 +645,30 @@ export function useSignalAllProcesses() {
     },
   });
 }
+
+// Process Stdin
+
+async function sendProcessStdin(name: string, chars: string): Promise<{ success: boolean; message: string }> {
+  const response = await fetch(`/api/supervisor/processes/${encodeURIComponent(name)}/stdin`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ chars }),
+  });
+  if (!response.ok) {
+    const error = await response.json();
+    throw new Error(error.error || 'Failed to send input');
+  }
+  return response.json();
+}
+
+export function useSendProcessStdin() {
+  return useMutation({
+    mutationFn: ({ name, chars }: { name: string; chars: string }) => sendProcessStdin(name, chars),
+    onSuccess: (data) => {
+      toast.success(data.message);
+    },
+    onError: (error: Error) => {
+      toast.error(`Failed to send input: ${error.message}`);
+    },
+  });
+}