MCP-Client fuer HTTP-Server: externe Werkzeuge durch dieselben Pruefstellen (#197)

- Core/Mcp/McpClient: Streamable HTTP (MCP 2025-06-18) - initialize, tools/list
  with paging, tools/call; answers as JSON or event stream; session kept and
  opened again when the server forgets it; answers capped at 4 MB.
- McpServerTool: one server is one tool (mcp_<id>); its remote tools are the
  operations, so rights, preflight, staging, lead approvals, votes, audit and
  budget apply. On approval by default unless the human trusts an operation;
  reads foreign content; the server's descriptions are cut short and marked.
- McpConnections: registers, refreshes every 15 minutes, follows changes.
- InstanceConfig.mcpServers with protected headers; /api/mcp-servers;
  settings card "External MCP servers"; tool catalog includes the servers.
- stdio servers are not part of this (sandbox runner, separate step).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ClawdDotNet.Manager.Web/wwwroot/app.css

@@ -2637,6 +2637,16 @@ button.org-einheit .org-name {
   gap: 4px;
 }
 
+/* #197: an external MCP server with its tools. */
+.mcp-server {
+  border-left: 3px solid var(--accent);
+  padding: 8px 12px;
+  margin-bottom: 12px;
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
 /* #691: a template before it comes in — every agent with its full texts. */
 .org-vorlage-agent {
   border-left: 3px solid var(--accent);