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/currentCard.js

@@ -0,0 +1,117 @@
+import Cards from '/models/cards';
+
+function getCardIdFromData(data) {
+  if (!data || !data._id) {
+    return null;
+  }
+
+  if (
+    data.boardId ||
+    data.listId ||
+    typeof data.absoluteUrl === 'function' ||
+    typeof data.getTitle === 'function'
+  ) {
+    return data._id;
+  }
+
+  return null;
+}
+
+function getCardIdFromElement(element) {
+  if (!element || typeof element.closest !== 'function') {
+    return null;
+  }
+
+  const cardDetails = element.closest('.js-card-details');
+  if (!cardDetails) {
+    return null;
+  }
+
+  return getCardIdFromData(Blaze.getData(cardDetails));
+}
+
+function getCardIdFromParentData(maxDepth = 8) {
+  for (let depth = 1; depth <= maxDepth; depth += 1) {
+    try {
+      const cardId = getCardIdFromData(Template.parentData(depth));
+      if (cardId) {
+        return cardId;
+      }
+    } catch (error) {
+      break;
+    }
+  }
+
+  return null;
+}
+
+function getPopupStack() {
+  if (typeof Popup !== 'undefined' && typeof Popup._getTopStack === 'function') {
+    return Popup._getTopStack();
+  }
+
+  return null;
+}
+
+export function getCurrentCardIdFromContext({ ignorePopupCard = false } = {}) {
+  let cardId;
+
+  try {
+    cardId = getCardIdFromData(Template.currentData());
+    if (cardId) {
+      return cardId;
+    }
+  } catch (error) {
+    // No active Blaze view.
+  }
+
+  cardId = getCardIdFromParentData();
+  if (cardId) {
+    return cardId;
+  }
+
+  const popupStack = getPopupStack();
+
+  cardId = getCardIdFromData(popupStack?.dataContext);
+  if (cardId) {
+    return cardId;
+  }
+
+  cardId = getCardIdFromElement(popupStack?.openerElement);
+  if (cardId) {
+    return cardId;
+  }
+
+  cardId = getCardIdFromElement(document.activeElement);
+  if (cardId) {
+    return cardId;
+  }
+
+  cardId = Session.get('currentCard');
+  if (cardId) {
+    return cardId;
+  }
+
+  if (!ignorePopupCard) {
+    cardId = Session.get('popupCardId');
+    if (cardId) {
+      return cardId;
+    }
+  }
+
+  const cardDetails = document.querySelectorAll('.js-card-details');
+  if (cardDetails.length === 1) {
+    return getCardIdFromElement(cardDetails[0]);
+  }
+
+  return null;
+}
+
+export function getCurrentCardFromContext(options) {
+  const cardId = getCurrentCardIdFromContext(options);
+  if (!cardId) {
+    return null;
+  }
+
+  return Cards.findOne(cardId);
+}