feat: Add MCP server icon support (SEP-973) v2.20.0

Implements custom server icons for MCP clients according to the MCP
specification SEP-973. Icons enable better visual identification of
the n8n-mcp server in MCP client interfaces.

Features:
- Added 3 icon sizes: 192x192, 128x128, 48x48 (PNG format)
- Icons served from https://www.n8n-mcp.com/logo*.png
- Added websiteUrl field pointing to https://n8n-mcp.com
- Server version now uses package.json (PROJECT_VERSION) instead of hardcoded '1.0.0'

Changes:
- Upgraded @modelcontextprotocol/sdk from ^1.13.2 to ^1.20.1
- Updated src/mcp/server.ts with icon configuration
- Bumped version to 2.20.0
- Updated CHANGELOG.md with release notes

Testing:
- All icon URLs verified accessible (HTTP 200, CORS enabled)
- Build passes, type checking passes
- No breaking changes, fully backward compatible

Icons won't display in Claude Desktop yet (pending upstream UI support),
but will appear automatically when support is added. Other MCP clients
may already support icon display.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.19.6",
+  "version": "2.20.0",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
@@ -139,7 +139,7 @@
     "vitest": "^3.2.4"
   },
   "dependencies": {
-    "@modelcontextprotocol/sdk": "^1.13.2",
+    "@modelcontextprotocol/sdk": "^1.20.1",
     "@n8n/n8n-nodes-langchain": "^1.114.1",
     "@supabase/supabase-js": "^2.57.4",
     "dotenv": "^16.5.0",