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/features/uploads/enqueue.dart

@@ -11,6 +11,11 @@
 /// state=uploaded + filename+bytes 匹配 → 返回 [DuplicateCheck]，由
 /// 调用方决定弹窗确认或跳过。批量场景汇总后单次弹窗（非逐张）。
 ///
+/// Task 2（查重前置到入队前·统一入口）：入队前查重升级为
+/// [checkDuplicatesUnified]——本机 TaskStore 历史 ∪ 服务器历史
+/// （duplicates/check 只读预检，bytes 命中）；服务器失败静默降级
+/// 本机-only，不阻塞入队。
+///
 /// Task 8（服务端查重 App 侧集成）：弹窗「全部上传」= 用户确认强传——
 /// 重复项以 force=true 入队（[enqueueForUpload] 的 forcePaths），任务行
 /// 落库后由引擎在 initUpload 携带 force 跳过服务端查重（仅 owner/admin
@@ -25,6 +30,8 @@ import 'package:image_picker/image_picker.dart';
 import 'package:path/path.dart' as p;
 import 'package:wechat_assets_picker/wechat_assets_picker.dart';
 
+import '../../core/api_client.dart'
+    show UploadDuplicateHit, apiClientProvider;
 import '../../core/notification_permission.dart';
 import 'providers.dart';
 import 'task_state.dart';
@@ -172,6 +179,77 @@ Future<List<DuplicateMatch>?> checkDuplicates(
   return matches.isEmpty ? null : matches;
 }
 
+/// 统一查重（P0-1 层 1+层 2 前置，Task 2）：本机 TaskStore 历史 ∪
+/// 服务器历史（POST /uploads/duplicates/check；服务端谓词仅 bytes——
+/// filename 刻意不参与，与 initUpload 查重一致）。
+///
+/// 服务器预检失败（网络/权限/任意异常）→ 静默降级本机-only（不阻塞
+/// 入队；服务端 init 409 仍是兜底）。返回 null = 无重复。
+///
+/// 服务器命中按全局下标映射回本批文件（stat 本批取 filename，与
+/// [checkDuplicates] 同模式），弹窗/强传标记的 filename 派生由此生效。
+Future<List<DuplicateMatch>?> checkDuplicatesUnified(
+  WidgetRef ref, {
+  required String accountId,
+  required String albumId,
+  required List<String> sourcePaths,
+}) async {
+  if (sourcePaths.isEmpty) return null;
+
+  // 层 1：本机 TaskStore 历史（filename+bytes 匹配）。
+  final local =
+      await checkDuplicates(
+        ref,
+        accountId: accountId,
+        albumId: albumId,
+        sourcePaths: sourcePaths,
+      ) ??
+      const <DuplicateMatch>[];
+
+  // 层 2：服务器历史（bytes 命中，逐请求条目一条）。
+  final picks = <({String filename, int bytes})>[];
+  for (final path in sourcePaths) {
+    try {
+      final file = File(path);
+      picks.add((filename: p.basename(path), bytes: await file.length()));
+    } on Object {
+      // 读不到的文件不参与查重（入队时准备器会报错）。
+    }
+  }
+  final serverHits = <UploadDuplicateHit>[];
+  if (picks.isNotEmpty) {
+    try {
+      serverHits.addAll(
+        await ref
+            .read(apiClientProvider)
+            .checkUploadDuplicates(albumId, picks),
+      );
+    } on Object {
+      // 静默降级本机-only：预检是体验优化而非闸门，失败不阻塞入队
+      //（服务端 init 409 仍是兜底）。
+    }
+  }
+
+  // 合并去重（filename+bytes；本机命中在前——同一重复保留先见者）。
+  final seen = <String>{};
+  final merged = <DuplicateMatch>[];
+  for (final match in [
+    ...local,
+    for (final hit in serverHits)
+      if (hit.index >= 0 && hit.index < picks.length)
+        DuplicateMatch(
+          filename: picks[hit.index].filename,
+          bytes: hit.bytes,
+          uploadedAt: hit.uploadedAt,
+        ),
+  ]) {
+    if (seen.add('${match.filename}\u0000${match.bytes}')) {
+      merged.add(match);
+    }
+  }
+  return merged.isEmpty ? null : merged;
+}
+
 /// 批量入队：通知权限单次请求 + 逐张准备入队。
 ///
 /// [skipPaths]：用户选择跳过的重复项；[forcePaths]：查重弹窗「全部上传」