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.

packages/shared/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@caldav/shared",
+  "name": "@calchat/shared",
   "version": "1.0.0",
   "private": true,
   "main": "./src/index.ts",