feat(app): server-side duplicate pre-check at enqueue moment

Task 2 of docs/plans/2026-09-28-上传查重前置到入队前-实施计划.md:
surface server-side duplicate history at the "click upload" moment,
merged with the local check into the EXISTING dialog.

- ApiClient.checkUploadDuplicates: POST /v1/uploads/duplicates/check
  (commit 805f77b endpoint); 500/chunk sequential requests; chunk-local
  index translated to global; empty list short-circuits (no request);
  errors normalized to AppException via _send.
- enqueue.checkDuplicatesUnified: local TaskStore history ∪ server
  history; server hits map back to filename via global index into the
  stat'd picks; merged, de-duplicated (filename+bytes) list; server
  failure (network/permission/any) silently degrades to local-only —
  init 409 remains the backstop.
- capture_page + album_detail_page _pickFromGallery now call the
  unified entry; dialog/skip/force logic untouched.

Tests: api_client chunking (1200 → 500/500/200) + index translation +
merge + 404 normalization + empty no-request; page-level three states
(hit/no-hit/network failure) incl. fresh-install server-only-hit dialog
on both pages; pre-existing Task 8 dialog tests keep passing via the
degrade path.

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

apps/mobile/lib/pages/album_detail_page.dart

@@ -342,8 +342,9 @@ class _AlbumDetailPageState extends ConsumerState<AlbumDetailPage> {
     if (paths.isEmpty || !mounted) return;
     final messenger = ScaffoldMessenger.of(context);
 
-    // P0-1 层 1：入队前查重。
-    final duplicates = await checkDuplicates(
+    // P0-1 层 1+2（Task 2 统一入口）：入队前查重——本机 TaskStore 历史
+    // ∪ 服务器历史（bytes 命中；服务器失败静默降级本机-only）。
+    final duplicates = await checkDuplicatesUnified(
       ref,
       accountId: account,
       albumId: album.id,

apps/mobile/lib/pages/capture_page.dart

@@ -453,8 +453,9 @@ class _CapturePageState extends ConsumerState<CapturePage>
     final l10n = AppLocalizations.of(context);
     final messenger = ScaffoldMessenger.of(context);
 
-    // P0-1 层 1：入队前查重（同相册已完成的 filename+bytes 匹配）。
-    final duplicates = await checkDuplicates(
+    // P0-1 层 1+2（Task 2 统一入口）：入队前查重——本机 TaskStore 历史
+    // ∪ 服务器历史（bytes 命中；服务器失败静默降级本机-only）。
+    final duplicates = await checkDuplicatesUnified(
       ref,
       accountId: widget.target.accountId,
       albumId: widget.target.albumId,