fix: show loading spinner when switching between contact threads

Clear old timeline items immediately on thread switch and display a centered
loader until the new conversation loads, instead of showing stale messages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/app/components/workspace/CrmWorkspaceApp.vue

@@ -259,6 +259,7 @@ const {
 // ---------------------------------------------------------------------------
 const {
   clientTimelineItems,
+  timelineLoading,
   timelineContactId,
   timelineLimit,
   loadClientTimeline,
@@ -1624,6 +1625,12 @@ onBeforeUnmount(() => {
 
                 <div v-else-if="selectedCommThread" class="relative flex h-full min-h-0 flex-col">
                   <div class="comm-thread-surface min-h-0 flex-1 space-y-2 overflow-y-auto px-3 pb-2">
+                    <!-- Loading spinner while timeline is fetching -->
+                    <div v-if="timelineLoading && clientTimelineItems.length === 0" class="flex h-full items-center justify-center">
+                      <span class="loading loading-spinner loading-md text-primary" />
+                    </div>
+
+                    <template v-else>
                     <button
                       class="sticky top-0 z-10 -mx-3 mb-2 flex w-[calc(100%+1.5rem)] items-center gap-2 border-b border-base-300 bg-base-100/80 px-3 py-2 text-left backdrop-blur-sm transition hover:bg-base-100"
                       @click="commPinnedOnly = !commPinnedOnly"
@@ -1900,6 +1907,7 @@ onBeforeUnmount(() => {
                         </div>
                       </div>
                     </div>
+                    </template>
                   </div>
 
                   <div