FIX: Resolve SSH session timeout and disconnection issues

Fixed SSH sessions being automatically removed after a few minutes of
inactivity, causing connection errors when users return to the interface.

## Problems Identified:

### 1. Aggressive Session Timeout
**Issue**: Sessions were cleaned up after only 10 minutes of inactivity
- Too short for typical user workflows
- No warning or graceful handling when timeout occurs
- Users would get connection errors without explanation

### 2. No Session Keepalive Mechanism
**Issue**: No frontend keepalive to maintain active sessions
- Sessions would timeout even if user was actively viewing files
- No periodic communication to extend session lifetime
- No way to detect session expiration proactively

### 3. Server-side SSH Configuration
**Issue**: While SSH had keepalive settings, they weren't sufficient
- keepaliveInterval: 30000ms (30s)
- keepaliveCountMax: 3
- But no application-level session management

## Technical Solution:

### **Extended Session Timeout**
```typescript
// Increased from 10 minutes to 30 minutes
session.timeout = setTimeout(() => {
  fileLogger.info(`Cleaning up inactive SSH session: ${sessionId}`);
  cleanupSession(sessionId);
}, 30 * 60 * 1000); // 30 minutes
```

### **Backend Keepalive Endpoint**
```typescript
// New endpoint: POST /ssh/file_manager/ssh/keepalive
app.post("/ssh/file_manager/ssh/keepalive", (req, res) => {
  const session = sshSessions[sessionId];
  session.lastActive = Date.now();
  scheduleSessionCleanup(sessionId); // Reset timeout
  res.json({ status: "success", connected: true });
});
```

### **Frontend Automatic Keepalive**
```typescript
// Send keepalive every 5 minutes
keepaliveTimerRef.current = setInterval(async () => {
  if (sshSessionId) {
    await keepSSHAlive(sshSessionId);
  }
}, 5 * 60 * 1000);
```

## Session Management Flow:

**Before (Problematic):**
1. User connects → 10-minute countdown starts
2. User leaves browser open but inactive
3. Session times out after 10 minutes
4. User returns → "SSH session not found" error
5. User forced to reconnect manually

**After (Fixed):**
1. User connects → 30-minute countdown starts
2. Frontend sends keepalive every 5 minutes automatically
3. Each keepalive resets the 30-minute timeout
4. Session stays alive as long as browser tab is open
5. Graceful handling if keepalive fails

## Benefits:
✅ **Extended Session Lifetime**: 30 minutes vs 10 minutes base timeout
✅ **Automatic Session Maintenance**: Keepalive every 5 minutes
✅ **Transparent to User**: No manual intervention required
✅ **Robust Error Handling**: Graceful degradation if keepalive fails
✅ **Resource Efficient**: Only active sessions consume resources
✅ **Better User Experience**: No unexpected disconnections

Sessions now persist for the entire duration users have the file
manager open, eliminating frustrating timeout errors.

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

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

src/backend/ssh/file-manager.ts

@@ -93,11 +93,11 @@ function scheduleSessionCleanup(sessionId: string) {
     // Clear existing timeout
     if (session.timeout) clearTimeout(session.timeout);
 
-    // Set new timeout for 10 minutes of inactivity
+    // Increase timeout to 30 minutes of inactivity
     session.timeout = setTimeout(() => {
       fileLogger.info(`Cleaning up inactive SSH session: ${sessionId}`);
       cleanupSession(sessionId);
-    }, 10 * 60 * 1000); // 10 minutes
+    }, 30 * 60 * 1000); // 30 minutes - increased from 10 minutes
   }
 }
 
@@ -321,6 +321,41 @@ app.get("/ssh/file_manager/ssh/status", (req, res) => {
   res.json({ status: "success", connected: isConnected });
 });
 
+// SSH keepalive endpoint - extends session timeout and verifies connection
+app.post("/ssh/file_manager/ssh/keepalive", (req, res) => {
+  const { sessionId } = req.body;
+
+  if (!sessionId) {
+    return res.status(400).json({ error: "Session ID is required" });
+  }
+
+  const session = sshSessions[sessionId];
+
+  if (!session || !session.isConnected) {
+    return res.status(400).json({
+      error: "SSH session not found or not connected",
+      connected: false
+    });
+  }
+
+  // Update last active time and reschedule cleanup
+  session.lastActive = Date.now();
+  scheduleSessionCleanup(sessionId);
+
+  fileLogger.debug(`SSH session keepalive: ${sessionId}`, {
+    operation: "ssh_keepalive",
+    sessionId,
+    lastActive: session.lastActive,
+  });
+
+  res.json({
+    status: "success",
+    connected: true,
+    message: "Session keepalive successful",
+    lastActive: session.lastActive
+  });
+});
+
 app.get("/ssh/file_manager/ssh/listFiles", (req, res) => {
   const sessionId = req.query.sessionId as string;
   const sshConn = sshSessions[sessionId];