[Bug][Windows][UX] No error message displayed when channel connection fails

[shangxinyu1]

Environment

Platform: Windows
Version: Latest (0.1.10 or recent nightly builds)
Affected Channels: All (WeChat, WhatsApp, Telegram, Discord, QQ, DingTalk)

Steps to Reproduce

1. Launch nexu on Windows
2. Attempt to connect a channel (e.g., WeChat, QQ, Telegram, etc.)
3. If the connection fails (due to network issues, authentication problems, or other errors)
4. Observe the UI response

Actual Behavior

When channel connection fails:

• No error message is displayed to the user
• The UI provides no feedback about why the connection failed
• Users are left uncertain whether:
  • The connection is still in progress
  • The connection has failed silently
  • There's an issue with their credentials/setup
• The channel status may remain stuck in "Connecting" or revert to "Disconnected" without explanation

Expected Behavior

When a channel connection fails, the application should:

1. Display a clear error message indicating the failure
2. Provide specific information about the error cause (e.g., "Authentication failed", "Network timeout", "Invalid QR code")
3. Suggest next steps for the user (e.g., "Please check your network connection and retry", "Re-scan QR code", "Verify credentials")
4. Show the error in a toast notification, modal dialog, or inline message
5. Log the error details for debugging purposes

Impact

Severity: P1 (High)

• Poor user experience - users don't know what went wrong
• Increased support burden - users can't self-diagnose connection issues
• Frustration when connections fail silently
• Users may assume the app is broken when it's actually a configuration/network issue

Suggested Fix

Add error handling and user-facing error messages for all channel connection failures:

• Network errors: "Unable to connect. Please check your internet connection."
• Authentication errors: "Login failed. Please verify your credentials."
• QR code errors: "QR code expired. Please scan again."
• Timeout errors: "Connection timeout. Please try again."
• Generic fallback: "Failed to connect to [Channel]. Error: [error message]"

Related Issues

• Agent and channels stuck in 'Starting/Connecting' state on Intel macOS (0.1.10-nightly) #934 (Intel macOS stuck in 'Starting/Connecting' state) - similar silent failure issue
• [Bug]QQ channel stuck in 'Restarting' state after app update #932 (QQ channel stuck in 'Restarting') - related to connection state management
• [Bug][UX] Browser confirmation page fails to auto-close after countdown reaches 0s #890 (Browser confirmation page auto-close issue) - another UX feedback gap