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/pages/t/[id].vue

@@ -69,16 +69,35 @@ const requestHost = useRequestURL().hostname
 const { user, can } = useAuth()
 const { t, locale } = useI18n()
 
+// Herkunftskontext aus dem State (kein URL-Param): gilt nur, wenn er für
+// genau diesen Thread gesetzt wurde — Tile-Klick oder weitergereichter
+// Pfeil. Der interne API-Call bekommt ihn als from-Param mit
+const navContext = useUploadNavContext()
+const activeContext = computed(() => navContext.value?.uploadId === id.value ? navContext.value : null)
+const apiFrom = computed(() => activeContext.value?.fromPath ? `?from=${encodeURIComponent(activeContext.value.fromPath)}` : '')
+
 // Beim Client-Side-Nav nicht awaiten, damit die Seite sofort rendert und das
 // benannte Medium-Element für die View-Transition im Snapshot vorhanden ist
 const fetchResult = useFetch<{ upload: UploadDetail, comments: CommentItem[], favoritedBy: FavUser[], isFavorited: boolean, isSubscribed: boolean, uploadReactions: UploadReaction[], newerId: string | null, olderId: string | null }>(
-  () => `/api/uploads/${id.value}`,
+  () => `/api/uploads/${id.value}${apiFrom.value}`,
 )
 if (import.meta.server) {
   await fetchResult
 }
 const { data, error, refresh } = fetchResult
 
+// Kontext für den Folgethread neu säen: uploadId wird auf das Ziel gesetzt,
+// fromPath/fromLabel wandern mit — so bleibt die Pfeil-Kette in der
+// Herkunftsliste, ohne dass der Param in der URL auftaucht
+function seedNavContext(nextId: string | null) {
+  if (!nextId) return
+  navContext.value = {
+    uploadId: nextId,
+    fromPath: activeContext.value?.fromPath ?? null,
+    fromLabel: activeContext.value?.fromLabel ?? null,
+  }
+}
+
 function setThreadSubscription(uploadId: string, enabled: boolean) {
   // useFetch-data ist ein shallowRef — nur eine Neuzuweisung triggert Reaktivität
   if (data.value?.upload.id === uploadId) {
@@ -598,7 +617,7 @@ async function deleteComment(commentId: number) {
             class="absolute left-2 top-1/2 -translate-y-1/2 shadow opacity-80 hover:opacity-100"
             as-child
           >
-            <NuxtLink :to="`/t/${data.newerId}`" :aria-label="$t('thread.newer')">
+            <NuxtLink :to="`/t/${data.newerId}`" :aria-label="$t('thread.newer')" @click="seedNavContext(data.newerId)">
               <Icon name="ph:caret-left-fill" class="size-5" />
             </NuxtLink>
           </Button>
@@ -609,7 +628,7 @@ async function deleteComment(commentId: number) {
             class="absolute right-2 top-1/2 -translate-y-1/2 shadow opacity-80 hover:opacity-100"
             as-child
           >
-            <NuxtLink :to="`/t/${data.olderId}`" :aria-label="$t('thread.older')">
+            <NuxtLink :to="`/t/${data.olderId}`" :aria-label="$t('thread.older')" @click="seedNavContext(data.olderId)">
               <Icon name="ph:caret-right-fill" class="size-5" />
             </NuxtLink>
           </Button>