Keep thread navigation within the originating upload list

Record where an upload was opened from in a new useUploadNavContext
state. When a tile is clicked on a user profile, the thread page passes
that context to the upload detail API, which scopes the newer/older
neighbors to that user's uploads. Arrow clicks reseed the context for
the next thread, so the chain stays within the source list without a
URL parameter. Threads opened directly (notification, reload) fall back
to the global order. The breadcrumb reads the same context and shows
the profile path for profile-originated threads, now including the
thread id in the final crumb.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/components/SiteBreadcrumb.vue

@@ -10,6 +10,7 @@ import {
 
 const { t } = useI18n()
 const route = useRoute()
+const navContext = useUploadNavContext()
 
 interface Crumb {
   label: string
@@ -20,7 +21,20 @@ const crumbs = computed<Crumb[]>(() => {
   const list: Crumb[] = [{ label: t('palette.home'), to: '/' }]
   const p = route.path
   if (p === '/') return []
-  if (p.startsWith('/t/')) list.push({ label: t('nav.thread') })
+  if (p.startsWith('/t/')) {
+    // Herkunft aus dem Nav-Kontext — wird von den Pfeilen weitergereicht.
+    // Gilt nur für die hinterlegte Upload-ID (Tile-Klick oder Pfeil-Seed);
+    // direkt geöffnete Threads (Notification, Reload) zeigen den schlichten
+    // Crumb ohne Profilkette
+    const ctx = navContext.value?.uploadId === route.params.id ? navContext.value : null
+    if (ctx?.fromPath) {
+      list.push(
+        { label: t('nav.users'), to: '/users' },
+        { label: ctx.fromLabel ?? t('nav.profile'), to: ctx.fromPath },
+      )
+    }
+    list.push({ label: `${t('nav.thread')} ${route.params.id ?? ''}` })
+  }
   else if (p === '/users') list.push({ label: t('nav.users') })
   else if (p.startsWith('/user/')) {
     list.push({ label: t('nav.users'), to: '/users' }, { label: t('nav.profile') })
@@ -41,9 +55,10 @@ const crumbs = computed<Crumb[]>(() => {
           <BreadcrumbLink v-if="crumb.to" as-child>
             <NuxtLink :to="crumb.to">{{ crumb.label }}</NuxtLink>
           </BreadcrumbLink>
-          <BreadcrumbPage v-else>
+          <BreadcrumbPage v-else-if="i === crumbs.length - 1">
             {{ crumb.label }}
           </BreadcrumbPage>
+          <span v-else>{{ crumb.label }}</span>
         </BreadcrumbItem>
         <BreadcrumbSeparator v-if="i < crumbs.length - 1" />
       </template>