hub v0.3.1: Config diff display + pull config

Replace broken SHA256 hash comparison with value-based YAML comparison.
Add "Show Diff" button showing per-key differences in a color-coded table.
Add "Pull Config" to import controller's current config into the Hub.
New endpoints: GET /customers/{id}/config-diff, POST /customers/{id}/pull-config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hub/internal/web/templates/style.css

@@ -613,6 +613,11 @@ code {
     color: #3b82f6;
 }
 
+/* Config diff table */
+.diff-changed td { color: #f59e0b; }
+.diff-hub_only td { color: #3b82f6; }
+.diff-controller_only td { color: #fb923c; }
+
 /* Responsive */
 @media (max-width: 768px) {
     .container { padding: 1rem; }