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>

Endpoints/ChatEndpoint.cs

@@ -292,7 +292,7 @@ public static class ChatEndpoint
 
                     if (anthropic.IsCliAvailable)
                     {
-                        await anthropic.StreamChatViaCli(res, ctx.ModelId, ctx.SystemPrompt, messages, ctx.ChainId, workspace: ctx.Workspace, allowUserChoice: ctx.Agent?.AllowUserChoice ?? true, planMode: body.PlanMode, ultracode: body.Ultracode, webSearch: !string.IsNullOrEmpty(ctx.WebSearchVersion), sessionId: sessionId, agentId: ctx.AgentId, remoteMgr: remoteMgr, unityMcpUrl: ctx.Session?.UnityMcpUrl, unityMcpToken: ctx.Session?.UnityMcpToken, useAutoContext: ctx.Agent?.UseAutoContext ?? true, effort: ctx.Effort, seedUsage: panelUsage, workCt: workCt, sseCt: sseCt);
+                        await anthropic.StreamChatViaCli(res, ctx.ModelId, ctx.SystemPrompt, messages, ctx.ChainId, workspace: ctx.Workspace, allowUserChoice: ctx.Agent?.AllowUserChoice ?? true, planMode: body.PlanMode, ultracode: body.Ultracode, webSearch: !string.IsNullOrEmpty(ctx.WebSearchVersion), sessionId: sessionId, agentId: ctx.AgentId, remoteMgr: remoteMgr, mcpServers: ctx.Session?.McpServers, useAutoContext: ctx.Agent?.UseAutoContext ?? true, effort: ctx.Effort, seedUsage: panelUsage, workCt: workCt, sseCt: sseCt);
                     }
                     else
                     {

Endpoints/SessionEndpoint.cs

@@ -83,6 +83,7 @@ public static class SessionEndpoint
                 s.PreparerAgentId,
                 s.UnityMcpUrl,
                 s.UnityMcpToken,
+                s.McpServers,
                 s.IsDeleted,
                 s.CreatedAt,
                 s.UpdatedAt,