feat: IPv6 support and multi-IP per node (closes #60)

- maskIp handles IPv6 addresses (masks second and last group)
- maskIp handles comma-separated IP strings (masks each address)
- Add splitIps() helper to parse comma-separated IP field
- Add primaryIp() helper used by status checker (first IP wins)
- BaseNode renders each IP on its own line when comma-separated
- NodeModal placeholder shows comma-separated example
- Backend status_checker uses only first IP for connectivity checks
- Expand maskIp test suite: IPv6, comma-separated, splitIps, primaryIp

frontend/src/components/canvas/__tests__/BaseNode.test.tsx

@@ -48,6 +48,7 @@ vi.mock('@/utils/nodeIcons', () => ({
 
 vi.mock('@/utils/maskIp', () => ({
   maskIp: (ip: string) => ip,
+  splitIps: (ip: string) => ip ? ip.split(',').map((s: string) => s.trim()).filter(Boolean) : [],
 }))
 
 vi.mock('@/utils/propertyIcons', () => ({