학생 계정 엑셀 일괄 등록 기능 추가 (신입생 등)

- 학생 계정 관리 화면에 드래그앤드롭/클릭 업로드 영역 추가
  (excel, desktop_drop, file_picker 패키지 도입)
- .xlsx 파싱 → 미리보기 팝업 → 확인 후 한 명씩 순서대로 계정 생성,
  결과(성공/실패)를 요약해서 보여줌. 초기 비밀번호는 1234 고정
- file_picker의 웹 구현체가 pickFile(s)를 아직 구현하지 않아서
  (UnimplementedError) 웹에서는 package:web으로 직접 <input type=file>을
  다뤄서 우회 (lib/function/excel_file_picker_web.dart), 다른 플랫폼은
  기존 file_picker 사용 (excel_file_picker_io.dart)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pubspec.lock

@@ -9,6 +9,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.3.75"
+  android_file_picker:
+    dependency: transitive
+    description:
+      name: android_file_picker
+      sha256: "1f111ed6bb33724ba782cc86357e3fd97f57051d55fc61adf33dcfc5049a1581"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.3"
+  archive:
+    dependency: transitive
+    description:
+      name: archive
+      sha256: cb6a278ef2dbb298455e1a713bda08524a175630ec643a242c399c932a0a1f7d
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.6.1"
   args:
     dependency: transitive
     description:
@@ -57,6 +73,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  cross_file:
+    dependency: transitive
+    description:
+      name: cross_file
+      sha256: f141ea4f277af142a0356955707f6556f37b03947d39d55585981a06ca437bd6
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.3.5+5"
+  crypto:
+    dependency: transitive
+    description:
+      name: crypto
+      sha256: c8ea0233063ba03258fbcf2ca4d6dadfefe14f02fab57702265467a19f27fadf
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.7"
   cupertino_icons:
     dependency: "direct main"
     description:
@@ -73,6 +105,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.7.14"
+  desktop_drop:
+    dependency: "direct main"
+    description:
+      name: desktop_drop
+      sha256: c65d1f959ccf4bb6803f984303d03e1e0dd4c0f73a3a2eca2d1b46d48ea16999
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.8.2"
   device_info_plus:
     dependency: "direct main"
     description:
@@ -89,6 +129,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "8.1.0"
+  equatable:
+    dependency: transitive
+    description:
+      name: equatable
+      sha256: "3bce007a596ff8b3119c45d68aaef631272537c03d30e5d4534dd24bf4c5eaa2"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.1.0"
+  excel:
+    dependency: "direct main"
+    description:
+      name: excel
+      sha256: "1a15327dcad260d5db21d1f6e04f04838109b39a2f6a84ea486ceda36e468780"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.0.6"
   fake_async:
     dependency: transitive
     description:
@@ -121,6 +177,46 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.1"
+  file_picker:
+    dependency: "direct main"
+    description:
+      name: file_picker
+      sha256: b7acb5d123cb398f6b4a8a38e43777545254f8fc1fabef3ee11cbbb56aece9c8
+      url: "https://pub.dev"
+    source: hosted
+    version: "12.1.2"
+  file_picker_darwin:
+    dependency: transitive
+    description:
+      name: file_picker_darwin
+      sha256: "6e7cae501d48fd57a27911608db718ebd898e6ccf934bf09e33a8c0d0365bec0"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.4"
+  file_picker_linux:
+    dependency: transitive
+    description:
+      name: file_picker_linux
+      sha256: f0f01ed42967b7355f6f25c8b121ea531d1948e2a9b4b44f4b4de8489d7b04ae
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.2"
+  file_picker_platform_interface:
+    dependency: transitive
+    description:
+      name: file_picker_platform_interface
+      sha256: "11ef1b5c14d9186b4788cc273dd8d5cb81bca847145060991a28234ff7916fc1"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.2.0"
+  file_picker_web:
+    dependency: transitive
+    description:
+      name: file_picker_web
+      sha256: df472142f63c4557fdfbb375c81454ca1c251491069eefcdc6a866bc12f8750b
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.3"
   firebase_core:
     dependency: "direct main"
     description:
@@ -517,6 +613,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.4.0"
+  universal_platform:
+    dependency: transitive
+    description:
+      name: universal_platform
+      sha256: "64e16458a0ea9b99260ceb5467a214c1f298d647c659af1bff6d3bf82536b1ec"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.0"
   vector_math:
     dependency: transitive
     description:
@@ -550,7 +654,7 @@ packages:
     source: hosted
     version: "15.2.0"
   web:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: web
       sha256: "868d88a33d8a87b18ffc05f9f030ba328ffefba92d6c127917a2ba740f9cfe4a"
@@ -573,6 +677,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.3"
+  windows_file_picker:
+    dependency: transitive
+    description:
+      name: windows_file_picker
+      sha256: "225f58e64c15c2d7b34fb8faf3ca188831967f26b5a723d7c3a7969e41c9b5a5"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.0"
   xdg_directories:
     dependency: transitive
     description: