feat(fc2a): implement Settings Import tab components

ImportTriggerPanel shows an active-batch progress indicator when scan_directory
is running, or a "Quick scan" button otherwise. Deep scan deferred to FC-2d
is called out in the panel text so it's not a surprise.

ImportFiltersForm autosaves on blur/change so there's no Submit button. The
transparency and single-color sliders gate on their respective switches.

ImportTaskList uses v-data-table-virtual for performance on long histories,
with status filter, retry-failed, and a clear-completed dialog with age
buckets. Status chips are color-coded via FabledDesignSystem semantic
tokens (success/warning/error/accent/info).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/ImportTriggerPanel.vue

@@ -1 +1,45 @@
-<template><div class="text-caption">ImportTriggerPanel — implemented in Task 15</div></template>
+<template>
+  <v-card>
+    <v-card-title>Trigger scan</v-card-title>
+    <v-card-text>
+      <div v-if="store.activeBatch" class="d-flex align-center" style="gap: 12px;">
+        <v-progress-circular
+          indeterminate color="accent" size="20"
+        />
+        <span>
+          Scanning {{ store.activeBatch.source_path }} —
+          imported {{ store.activeBatch.imported }},
+          skipped {{ store.activeBatch.skipped }},
+          failed {{ store.activeBatch.failed }} /
+          {{ store.activeBatch.total_files }} files
+        </span>
+      </div>
+      <div v-else>
+        <p class="text-body-2 mb-3">
+          Run a quick scan of the import directory. Deep scan (pHash dedup,
+          archives) lands in FC-2d.
+        </p>
+        <v-btn color="primary" rounded="pill" @click="trigger" :loading="busy">
+          <v-icon start>mdi-magnify-scan</v-icon>
+          Quick scan
+        </v-btn>
+        <v-alert v-if="store.triggerError" type="error" variant="tonal" class="mt-3" closable>
+          {{ store.triggerError }}
+        </v-alert>
+      </div>
+    </v-card-text>
+  </v-card>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+import { useImportStore } from '../../stores/import.js'
+
+const store = useImportStore()
+const busy = ref(false)
+
+async function trigger() {
+  busy.value = true
+  try { await store.triggerScan() } catch {} finally { busy.value = false }
+}
+</script>