refactor: rename package scope from @caldav to @calchat

Rename all workspace packages to reflect the actual project name:
- @caldav/client -> @calchat/client
- @caldav/server -> @calchat/server
- @caldav/shared -> @calchat/shared
- Root package: caldav-mono -> calchat-mono

Update all import statements across client and server to use the new
package names. Update default MongoDB database name and logging service
identifier accordingly.

apps/client/app.json

@@ -2,10 +2,10 @@
   "expo": {
     "jsEngine": "hermes",
     "name": "CalChat",
-    "slug": "caldav",
+    "slug": "calchat",
     "version": "1.0.0",
     "orientation": "portrait",
-    "scheme": "caldav",
+    "scheme": "calchat",
     "userInterfaceStyle": "automatic",
     "newArchEnabled": true,
     "ios": {