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/ClaudeCodeStyles.css

@@ -287,6 +287,11 @@
   /* Tool Call Styles */
   --app-tool-background: var(--vscode-editor-background);
   --app-code-background: var(--vscode-textCodeBlock-background);
+
+  /* Warning/Error Styles */
+  --app-warning-background: var(--vscode-editorWarning-background, rgba(255, 204, 0, 0.1));
+  --app-warning-border: var(--vscode-editorWarning-foreground, #ffcc00);
+  --app-warning-foreground: var(--vscode-editorWarning-foreground, #ffcc00);
 }
 
 /* ===========================