feat(ui): add exclude superseded failures filter toggle to Downloads

Adds a new filter toggle to the Downloads view that allows users to hide
superseded failed downloads. The filter is enabled by default and passes
exclude_superseded=true to the API when active.

Changes:
- Add filterExcludeSuperseded ref initialized to true
- Add filterExcludeSuperseded to watch dependencies
- Include exclude_superseded param in loadDownloads() when true
- Add v-switch toggle UI control below date filters

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

frontend/src/views/Downloads.vue

@@ -112,6 +112,19 @@
       </v-col>
     </v-row>
 
+    <!-- Superseded filter -->
+    <v-row class="mb-4">
+      <v-col cols="12">
+        <v-switch
+          v-model="filterExcludeSuperseded"
+          label="Hide superseded failures"
+          density="compact"
+          hide-details
+          color="primary"
+        />
+      </v-col>
+    </v-row>
+
     <!-- Downloads Table -->
     <v-card>
       <v-data-table
@@ -300,6 +313,7 @@ const filterStatus = ref(null)
 const filterSourceId = ref(null)
 const filterFromDate = ref(null)
 const filterToDate = ref(null)
+const filterExcludeSuperseded = ref(true)
 const detailsDialog = ref(false)
 const selectedDownload = ref(null)
 const retryingIds = ref([])
@@ -350,7 +364,7 @@ onUnmounted(() => {
   }
 })
 
-watch([filterStatus, filterSourceId, filterFromDate, filterToDate], () => {
+watch([filterStatus, filterSourceId, filterFromDate, filterToDate, filterExcludeSuperseded], () => {
   loadDownloads()
 })
 
@@ -367,6 +381,7 @@ async function loadDownloads() {
   if (filterSourceId.value) params.source_id = filterSourceId.value
   if (filterFromDate.value) params.from_date = filterFromDate.value
   if (filterToDate.value) params.to_date = filterToDate.value
+  if (filterExcludeSuperseded.value) params.exclude_superseded = true
   await downloadsStore.fetchDownloads(params)
 }