feat(vscode-ide-companion): implement manual login via /login command

BREAKING CHANGE: Login is no longer automatic when opening webview

Changes:
- Remove auto-login on webview open and restore
- Add /login slash command for manual authentication
- Add VSCode progress notification during login process
- Add warning notification when user tries to chat without login
- Implement pending message auto-retry after successful login
- Add NotLoggedInMessage component (for future use)
- Improve InfoBanner close button styling consistency

User flow:
1. Open webview - no automatic login
2. Type /login or select from completion menu to login
3. Show "Logging in to Qwen Code..." progress notification
4. After login, show success message and auto-retry pending messages
5. If user tries to chat without login, show warning with "Login Now" button

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

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

packages/vscode-ide-companion/src/webview/components/InfoBanner.tsx

@@ -98,11 +98,13 @@ export const InfoBanner: React.FC<InfoBannerProps> = ({
 
       {/* Close button */}
       <button
-        className="flex items-center justify-center cursor-pointer p-1.5 rounded"
+        className="flex items-center justify-center cursor-pointer rounded"
         style={{
           background: 'none',
           border: 'none',
+          padding: '6px',
           color: 'var(--app-secondary-foreground)',
+          borderRadius: '4px',
         }}
         onMouseEnter={(e) => {
           e.currentTarget.style.backgroundColor =