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/server/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@caldav/server",
+  "name": "@calchat/server",
   "version": "1.0.0",
   "private": true,
   "scripts": {
@@ -8,7 +8,7 @@
     "start": "node dist/app.js"
   },
   "dependencies": {
-    "@caldav/shared": "*",
+    "@calchat/shared": "*",
     "bcrypt": "^6.0.0",
     "dotenv": "^16.4.7",
     "express": "^5.2.1",