feat: add Drone CI pipelines, Jest unit tests, and Prettier check

Add Drone CI with server build/test and format check pipelines.
Add unit tests for password utils and recurrenceExpander.
Add check_format script, fix Jest config to ignore dist/,
remove dead CaldavService.test.ts, apply Prettier formatting.

.drone.yml

@@ -0,0 +1,31 @@
+---
+
+kind: pipeline
+name: server_build_and_test
+
+steps:
+  - name: build_server
+    image: node
+    commands:
+      - npm ci -w @calchat/shared
+      - npm ci -w @calchat/server
+      - npm run build -w @calchat/server
+
+  - name: jest_server
+    image: node
+    commands:
+      - npm run test -w @calchat/server
+
+---
+
+kind: pipeline
+name: check_for_formatting
+
+steps:
+  - name: format_check
+    image: node
+    commands:
+      - npm ci
+      - npm run check_format
+
+---