Generalize MCP servers, add model refresh button, add sonnet-5

- MCP: replace fixed single Unity field with a generic list of servers
  ({name, url, token?}); dynamic mcp-config + allowlist glob mcp__*<name>*
  per entry; new mcp-servers-ui.js editor (no data migration)
- Models: frontend refresh button wired to existing POST /api/models/refresh,
  reloads optgroups and restores selection
- Model: add sonnet-5 alias + dropdown option (bare "sonnet" stays 4.6)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Services/ChatService.cs

@@ -68,6 +68,7 @@ Nur deine finale Text-Antwort wird weiterverwendet. Formuliere sie vollständig
             "sonnet" => "claude-sonnet-4-6",
             "sonnet-4-5" => "claude-sonnet-4-5",
             "sonnet-4-6" => "claude-sonnet-4-6",
+            "sonnet-5" => "claude-sonnet-5",
             "opus" => "claude-opus-4-8",
             "opus-4-5" => "claude-opus-4-5",
             "opus-4-6" => "claude-opus-4-6",