Fix: Traefik proxy startup issues - handle null versions and filter predefined networks

Fixes two critical issues preventing Traefik proxy startup:

1. TypeError when restarting proxy: Handle null return from get_traefik_versions()
   - Add null check before dispatching CheckTraefikVersionForServerJob
   - Log warning when version data is unavailable
   - Prevents: "Argument #2 must be of type array, null given"

2. Docker network error: Filter out predefined Docker networks
   - Add isDockerPredefinedNetwork() helper to centralize network filtering
   - Apply filtering in collectDockerNetworksByServer() before operations
   - Apply filtering in generateDefaultProxyConfiguration()
   - Prevents: "operation is not permitted on predefined default network"

Also: Move $cachedVersionsFile assignment after null check in Proxy.php

Tests: Added 7 new unit tests for network filtering function
All existing tests pass with no regressions

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

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

app/Livewire/Server/Proxy.php

@@ -79,12 +79,11 @@ class Proxy extends Component
 
         // Load from global cached helper (Redis + filesystem)
         $versionsData = get_versions_data();
-        $this->cachedVersionsFile = $versionsData;
-
         if (! $versionsData) {
             return null;
         }
 
+        $this->cachedVersionsFile = $versionsData;
         $traefikVersions = data_get($versionsData, 'traefik');
 
         return is_array($traefikVersions) ? $traefikVersions : null;