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());

client/components/mixins/infiniteScrolling.js

@@ -1,34 +0,0 @@
-const peakAnticipation = 200;
-
-Mixins.InfiniteScrolling = BlazeComponent.extendComponent({
-  onCreated() {
-    this._nextPeak = Infinity;
-  },
-
-  setNextPeak(v) {
-    this._nextPeak = v;
-  },
-
-  getNextPeak() {
-    return this._nextPeak;
-  },
-
-  resetNextPeak() {
-    this._nextPeak = Infinity;
-  },
-
-  events() {
-    return [
-      {
-        scroll(evt) {
-          const domElement = evt.currentTarget;
-          let altitude = domElement.scrollTop + domElement.offsetHeight;
-          altitude += peakAnticipation;
-          if (altitude >= this.callFirstWith(null, 'getNextPeak')) {
-            this.mixinParent().callFirstWith(null, 'reachNextPeak');
-          }
-        },
-      },
-    ];
-  },
-});