feat(ui): use autocomplete for Downloads Source filter

Swaps v-select for v-autocomplete on the Source filter so users can
type-to-filter instead of scrolling a long list. Same data source, same
selection semantics.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/views/Downloads.vue

@@ -91,7 +91,7 @@
             class="mb-2"
             hide-details
           />
-          <v-select
+          <v-autocomplete
             v-model="filterSourceId"
             :items="sourceOptions"
             label="Source"
@@ -100,6 +100,7 @@
             clearable
             class="mb-2"
             hide-details
+            no-data-text="No matching source"
           />
           <v-text-field
             v-model="filterFromDate"