chore: split plugin configuration templates

internal/admin/templates/plugins/security.domainblock.html

@@ -0,0 +1,12 @@
+{{define "plugins/security.domainblock.html"}}
+<div class="mb-3">
+    <label class="form-label">Blocked Domains</label>
+    <input type="text" class="form-control" name="blocked_domains"
+           value="{{with .ChannelPlugin.Config}}{{index . "blocked_domains"}}{{end}}"
+           placeholder="example.com, evil.org, ads.com">
+    <div class="form-text text-muted">
+        Enter comma-separated list of domains to block (e.g., example.com, evil.org).
+        Messages containing links to these domains will be blocked.
+    </div>
+</div>
+{{end}}