新增全局协作台账并统一工作台管理界面

新增与项目平级的协作台账，提供空白模板、独立账号授权和在线编辑。
复用共享库的访问权限与历史版本界面，拆分信息维护入口并统一工作台卡片样式。
补充在线文档字体构建支持、数据库表结构升级和回归测试。
不包含本地台账内容、账号授权记录或历史文件迁移数据。

backend/app/models/collaboration.py

@@ -4,7 +4,7 @@ import uuid
 from datetime import datetime
 from typing import Optional
 
-from sqlalchemy import BigInteger, Boolean, DateTime, ForeignKey, Index, Integer, String, Text, UniqueConstraint, func, text
+from sqlalchemy import BigInteger, Boolean, CheckConstraint, DateTime, ForeignKey, Index, Integer, String, Text, UniqueConstraint, func, text
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -35,12 +35,22 @@ class CollaborationFolder(Base):
 class CollaborationFile(Base):
     __tablename__ = "collaboration_files"
     __table_args__ = (
+        CheckConstraint(
+            "(scope = 'PROJECT' AND study_id IS NOT NULL AND ledger_key IS NULL) OR "
+            "(scope = 'LEDGER' AND study_id IS NULL AND folder_id IS NULL AND ledger_key IS NOT NULL "
+            "AND file_type = 'cell' AND extension = 'xlsx' AND deleted_at IS NULL AND status IN ('ACTIVE', 'ARCHIVED'))",
+            name="ck_collaboration_file_scope",
+        ),
+        UniqueConstraint("ledger_key", name="uq_collaboration_file_ledger_key"),
         Index("ix_collaboration_files_study_folder", "study_id", "folder_id"),
         Index("ix_collaboration_files_study_status", "study_id", "status"),
     )
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
-    study_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("studies.id"), nullable=False)
+    study_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("studies.id"), nullable=True)
+    scope: Mapped[str] = mapped_column(String(16), nullable=False, default="PROJECT", server_default="PROJECT")
+    ledger_key: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
+    description: Mapped[str] = mapped_column(String(500), nullable=False, default="", server_default="")
     folder_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True), ForeignKey("collaboration_folders.id", ondelete="SET NULL"), nullable=True
     )