Add Hub Simple home with slash-command parser and header/sidebar navigation tweaks

[esafwan]

Motivation

• Provide a simplified "Hub Simple" home experience focused on quick natural-language and slash-command workflows instead of the previous metrics/tabs dashboard.
• Expose an easy path to the existing Advanced Hub and chat interfaces from the home header and pages.
• Allow routing behavior to control sidebar default open state so the app is expanded for non-root pages.

Description

• Reworked HomePage to replace the metrics/tabs dashboard with a command-first UI, starter prompts, command picker, and informational cards, and added navigation buttons to '/agents' and '/chat'.
• Added a new command parser service in frontend/src/services/commandParser.ts exporting parseSlashCommand and related types to parse slash-style commands and qualifiers.
• Updated HomeHeaderActions to add an inline "Switch to Advanced Hub" button and include an Open Advanced Hub entry in the new dropdown separated by DropdownMenuSeparator.
• Modified UnifiedLayout to compute defaultOpen from useLocation() and pass it to SidebarProvider so the sidebar opens by default for non-root routes.

Testing

• Ran TypeScript type-check (tsc --noEmit) against the frontend codebase and it completed successfully.
• Performed a local frontend build and dev start check and the app compiled and loaded without build-time errors.
• Exercised parseSlashCommand manually via the UI flow and verified parsing and routing for sample commands succeeded; no automated unit tests were added in this change.

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Codex Task