feat: implement XMPP bridge configuration and logger setup

- Add comprehensive XMPP settings schema to plugin.json
- Implement configuration struct with validation and helper methods
- Add configurable username prefix for XMPP users
- Set up logger in Plugin struct following Matrix bridge pattern
- Update KV store constants to use XMPP terminology
- Replace Matrix references with XMPP equivalents in test helpers

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

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

plugin.json

@@ -21,10 +21,53 @@
     "bundle_path": "webapp/dist/main.js"
   },
   "settings_schema": {
-    "header": "",
-    "footer": "",
-    "settings": [],
-    "sections": null
+    "header": "Configure the XMPP bridge connection settings below.",
+    "footer": "For more information about setting up the XMPP bridge, see the [documentation](https://github.com/mattermost/mattermost-plugin-bridge-xmpp/blob/main/README.md).",
+    "settings": [
+      {
+        "key": "XMPPServerURL",
+        "display_name": "XMPP Server URL",
+        "type": "text",
+        "help_text": "The URL of the XMPP server to connect to (e.g., xmpp.example.com:5222)",
+        "placeholder": "xmpp.example.com:5222"
+      },
+      {
+        "key": "XMPPUsername",
+        "display_name": "XMPP Username",
+        "type": "text",
+        "help_text": "The username for authenticating with the XMPP server",
+        "placeholder": "bridge@xmpp.example.com"
+      },
+      {
+        "key": "XMPPPassword",
+        "display_name": "XMPP Password",
+        "type": "password",
+        "help_text": "The password for authenticating with the XMPP server"
+      },
+      {
+        "key": "EnableSync",
+        "display_name": "Enable Message Synchronization",
+        "type": "bool",
+        "help_text": "When enabled, messages will be synchronized between Mattermost and XMPP",
+        "default": false
+      },
+      {
+        "key": "XMPPUsernamePrefix",
+        "display_name": "XMPP Username Prefix",
+        "type": "text",
+        "help_text": "Prefix for XMPP users in Mattermost (e.g., 'xmpp' creates usernames like 'xmpp:user@domain')",
+        "placeholder": "xmpp",
+        "default": "xmpp"
+      },
+      {
+        "key": "XMPPResource",
+        "display_name": "XMPP Resource",
+        "type": "text",
+        "help_text": "XMPP resource identifier for the bridge client",
+        "placeholder": "mattermost-bridge",
+        "default": "mattermost-bridge"
+      }
+    ]
   },
   "props": {
     "pluginctl": {