fix(settings): sticky headers on the virtual data tables

Allowlist / Alias / ImportTask tables scroll their bodies (height=360/480) but
the column headers scrolled away with the rows, so you lost the column labels
(operator-flagged 2026-06-27). Add Vuetify `fixed-header` so the header row
stays pinned while the body scrolls.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/AliasTable.vue

@@ -6,7 +6,7 @@
   >
     <v-data-table-virtual
       :headers="headers" :items="store.rows" :loading="store.loading"
-      height="360" density="compact"
+      height="360" density="compact" fixed-header
       no-data-text="No aliases yet — create one from a suggestion's ⋮ menu."
     >
       <template #item.mapping="{ item }">

frontend/src/components/settings/AllowlistTable.vue

@@ -7,7 +7,7 @@
   >
     <v-data-table-virtual
       :headers="headers" :items="store.rows" :loading="store.loading"
-      height="360" density="compact"
+      height="360" density="compact" fixed-header
       no-data-text="No tags on the allowlist yet — accept a suggestion to add one."
     >
       <template #item.applied_count="{ item }">

frontend/src/components/settings/ImportTaskList.vue

@@ -31,7 +31,7 @@
     </CardHeading>
     <v-data-table-virtual
       :headers="headers" :items="store.tasks" :loading="store.tasksLoading"
-      height="480" density="compact" no-data-text="No tasks yet — trigger a scan above."
+      height="480" density="compact" fixed-header no-data-text="No tasks yet — trigger a scan above."
     >
       <template #item.status="{ item }">
         <v-chip :color="statusColor(item.status)" size="small" variant="tonal">