fix: ensure danger modal appears on top of checkpoint modal

Issue:
- Restore checkpoint confirmation dialog (dangerModal) was hidden behind checkpoint management modal
- z-index of dangerModal (1000) was same as checkpointModal, causing layering issues

Solution:
- Increased z-index of .modal (dangerModal) from 1000 to 9999
- Now危险操作 confirmation dialog always appears on top of other modals
- Users can now properly see and interact with restore confirmation

This ensures critical operation confirmations are always visible and accessible.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

static/db_admin.html

@@ -445,7 +445,7 @@
             width: 100%;
             height: 100%;
             background: rgba(0, 0, 0, 0.5);
-            z-index: 1000;
+            z-index: 9999;
             align-items: center;
             justify-content: center;
         }