Fix issue #142: Implement integrated folder management in Host Viewer

Problem: Users had to rename folders on each host individually, which was
time-consuming and error-prone when managing multiple hosts in folders.

Solution: Complete folder management integration directly in Host Viewer with:

**Core Features:**
- Remove folder buttons: Hosts in folders show "X" buttons to move them to "no folder"
- Inline folder editing: Click folder names to edit directly (with pencil icon hint)
- Drag & drop: Drag host cards between folders with visual feedback
- Centralized renaming: Rename folders once, automatically updates all hosts

**UI/UX Enhancements:**
- Visual feedback: Drag states, hover effects, loading indicators
- Smart buttons: Only show remove buttons for hosts actually in folders
- Confirmation dialogs: Prevent accidental operations
- Real-time updates: Immediate UI refresh after operations
- Touch-friendly: Works well with drag & drop on different devices

**Technical Implementation:**
- HTML5 drag & drop API for cross-browser compatibility
- Integrated with existing Host Viewer without separate pages
- Reuses backend APIs for folder operations and host updates
- Comprehensive error handling and user feedback
- Event-driven updates for seamless state synchronization

**Backend Support:**
- Added folder statistics API: GET /ssh/db/folders/with-stats
- Added bulk rename API: PUT /ssh/db/folders/rename
- Fixed encryption key issue: Added CREDENTIAL_ENCRYPTION_KEY to .env

**Removed Components:**
- Eliminated separate "Folder Manager" tab for cleaner UX
- FolderManager.tsx kept for potential future use but not integrated

**Internationalization:**
- Complete English/Chinese support for all new features
- Contextual messages with dynamic content (host names, folder names)

This solution transforms folder management from a tedious multi-step process
into an intuitive, visual experience integrated directly into the main host
management workflow.

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

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

public/locales/en/translation.json

@@ -431,7 +431,14 @@
     "terminal": "Terminal",
     "tunnel": "Tunnel",
     "fileManager": "File Manager",
-    "hostViewer": "Host Viewer"
+    "hostViewer": "Host Viewer",
+    "confirmRemoveFromFolder": "Are you sure you want to remove \"{{name}}\" from folder \"{{folder}}\"? The host will be moved to \"No Folder\".",
+    "removedFromFolder": "Host \"{{name}}\" removed from folder successfully",
+    "failedToRemoveFromFolder": "Failed to remove host from folder",
+    "folderRenamed": "Folder \"{{oldName}}\" renamed to \"{{newName}}\" successfully",
+    "failedToRenameFolder": "Failed to rename folder",
+    "movedToFolder": "Host \"{{name}}\" moved to \"{{folder}}\" successfully",
+    "failedToMoveToFolder": "Failed to move host to folder"
   },
   "terminal": {
     "title": "Terminal",