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/lib/infiniteScrolling.js

@@ -0,0 +1,47 @@
+const PEAK_ANTICIPATION = 200;
+
+/**
+ * Infinite scrolling utility to replace the BlazeComponent mixin.
+ *
+ * Usage in a Template:
+ *   Template.myTemplate.onCreated(function () {
+ *     this.infiniteScrolling = new InfiniteScrolling();
+ *   });
+ *
+ * The scroll event must be wired in Template.events:
+ *   'scroll .my-container'(event, tpl) {
+ *     tpl.infiniteScrolling.checkScrollPosition(event.currentTarget, () => {
+ *       tpl.loadNextPage();
+ *     });
+ *   },
+ *
+ * Or for components that delegate to a child for loading:
+ *   tpl.infiniteScrolling.checkScrollPosition(event.currentTarget, () => {
+ *     activitiesComponent.loadNextPage();
+ *   });
+ */
+export class InfiniteScrolling {
+  constructor() {
+    this._nextPeak = Infinity;
+  }
+
+  setNextPeak(v) {
+    this._nextPeak = v;
+  }
+
+  getNextPeak() {
+    return this._nextPeak;
+  }
+
+  resetNextPeak() {
+    this._nextPeak = Infinity;
+  }
+
+  checkScrollPosition(domElement, reachNextPeakCallback) {
+    let altitude = domElement.scrollTop + domElement.offsetHeight;
+    altitude += PEAK_ANTICIPATION;
+    if (altitude >= this._nextPeak) {
+      reachNextPeakCallback();
+    }
+  }
+}