Replace templates:tabs Atmosphere package with inline Blaze implementation

Drop the templates:tabs dependency as part of Meteor 3.0 migration prep.
The package is replaced by a lightweight inline basicTabs/tabContent
template pair with identical API, so no consumer template changes needed.

client/components/lib/basicTabs.js

@@ -0,0 +1,50 @@
+const { ReactiveVar } = require('meteor/reactive-var');
+
+Template.basicTabs.onCreated(function () {
+  const activeTab = this.data.activeTab
+    ? { slug: this.data.activeTab }
+    : this.data.tabs[0];
+  this._activeTab = new ReactiveVar(activeTab);
+
+  this.isActiveSlug = (slug) => {
+    const current = this._activeTab.get();
+    return current && current.slug === slug;
+  };
+});
+
+Template.basicTabs.helpers({
+  isActiveTab(slug) {
+    if (Template.instance().isActiveSlug(slug)) {
+      return 'active';
+    }
+  },
+});
+
+Template.basicTabs.events({
+  'click .tab-item'(e, t) {
+    t._activeTab.set(this);
+  },
+});
+
+function findBasicTabsInstance() {
+  let view = Blaze.currentView;
+  while (view) {
+    if (view.name === 'Template.basicTabs' && view.templateInstance) {
+      const inst = view.templateInstance();
+      if (inst && inst.isActiveSlug) {
+        return inst;
+      }
+    }
+    view = view.parentView;
+  }
+  return null;
+}
+
+Template.tabContent.helpers({
+  isActiveTab(slug) {
+    const inst = findBasicTabsInstance();
+    if (inst && inst.isActiveSlug(slug)) {
+      return 'active';
+    }
+  },
+});