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>

app/api/supervisor/processes/[name]/stdin/route.ts

@@ -0,0 +1,36 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { createSupervisorClient } from '@/lib/supervisor/client';
+
+interface RouteParams {
+  params: Promise<{ name: string }>;
+}
+
+// POST - Send input to process stdin
+export async function POST(request: NextRequest, { params }: RouteParams) {
+  try {
+    const { name } = await params;
+    const body = await request.json();
+    const { chars } = body;
+
+    if (chars === undefined || chars === null) {
+      return NextResponse.json(
+        { error: 'Input characters are required' },
+        { status: 400 }
+      );
+    }
+
+    const client = createSupervisorClient();
+    const result = await client.sendProcessStdin(name, chars);
+
+    return NextResponse.json({
+      success: result,
+      message: `Input sent to ${name}`,
+    });
+  } catch (error: any) {
+    console.error('Supervisor send stdin error:', error);
+    return NextResponse.json(
+      { error: error.message || 'Failed to send input to process' },
+      { status: 500 }
+    );
+  }
+}