Replace BlazeComponent mixins with standalone utility modules

Delete the old Mixins global and BlazeComponent-based InfiniteScrolling
mixin. Add standalone replacements:
- client/lib/infiniteScrolling.js: plain scroll-based pagination helper
- client/lib/currentCard.js: centralized current card resolution
- client/components/forms/datepicker.js: extracted date picker logic

client/components/forms/datepicker.js

@@ -0,0 +1,16 @@
+import {
+  setupDatePicker,
+  datePickerRendered,
+  datePickerHelpers,
+  datePickerEvents,
+} from '/client/lib/datepicker';
+
+Template.datepicker.onCreated(function () {
+  setupDatePicker(this);
+});
+
+Template.datepicker.onRendered(function () {
+  datePickerRendered(this);
+});
+
+Template.datepicker.helpers(datePickerHelpers());