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>

components/process/ProcessCard.tsx

@@ -1,13 +1,14 @@
 'use client';
 
 import { useState } from 'react';
-import { Play, Square, RotateCw, Activity, Zap } from 'lucide-react';
+import { Play, Square, RotateCw, Activity, Zap, Terminal } from 'lucide-react';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
 import { Button } from '@/components/ui/button';
 import { Badge } from '@/components/ui/badge';
 import { ProcessInfo, ProcessStateCode, getProcessStateClass, formatUptime, canStartProcess, canStopProcess } from '@/lib/supervisor/types';
 import { useStartProcess, useStopProcess, useRestartProcess } from '@/lib/hooks/useSupervisor';
 import { SignalSender } from './SignalSender';
+import { StdinInput } from './StdinInput';
 import { cn } from '@/lib/utils/cn';
 
 interface ProcessCardProps {
@@ -18,6 +19,7 @@ interface ProcessCardProps {
 
 export function ProcessCard({ process, isSelected = false, onSelectionChange }: ProcessCardProps) {
   const [showSignalModal, setShowSignalModal] = useState(false);
+  const [showStdinModal, setShowStdinModal] = useState(false);
   const startMutation = useStartProcess();
   const stopMutation = useStopProcess();
   const restartMutation = useRestartProcess();
@@ -143,6 +145,15 @@ export function ProcessCard({ process, isSelected = false, onSelectionChange }:
           >
             <Zap className="h-4 w-4" />
           </Button>
+          <Button
+            size="sm"
+            variant="outline"
+            onClick={() => setShowStdinModal(true)}
+            disabled={process.state !== 20}
+            title="Send Stdin"
+          >
+            <Terminal className="h-4 w-4" />
+          </Button>
         </div>
 
         {/* Description */}
@@ -157,6 +168,11 @@ export function ProcessCard({ process, isSelected = false, onSelectionChange }:
       {showSignalModal && (
         <SignalSender processName={fullName} onClose={() => setShowSignalModal(false)} />
       )}
+
+      {/* Stdin Modal */}
+      {showStdinModal && (
+        <StdinInput processName={fullName} onClose={() => setShowStdinModal(false)} />
+      )}
     </Card>
   );
 }