v0.1.1: refactor: consolidate chat UI views and add sidebar/message ViewModels

SPLA v0.1.1

Chat UI overhaul and Network plugin skills.

Changed

• Chat rendering — unified view replacing the previous Bubble / Classic / Diagnostic stream views; WebView renderer extended with Markdown and Mermaid diagram support
• Message ViewModels — split into typed classes (User, Assistant, System) instead of a single generic ViewModel
• Sidebar — new SidebarPanelView with its own ViewModel and node hierarchy
• Settings — ChatDisplayProfile type added; SettingsResolver and SplaSections extended to cover display profiles
• Density themes — all four density files (Nano / Mini / Norm / Max) updated for the new layout

Added

• Network plugin skills — five agent skill definitions shipped with the Network plugin:
  • dns-diagnostics — DNS resolution and propagation checks
  • host-audit — per-host reachability and port survey
  • lan-discovery — local network device discovery via ARP
  • smtp-probe — mail server connectivity and banner checks
  • ssl-audit — TLS certificate inspection