Add guest mode functionality for read-only access

- Implement guest mode for unauthenticated users
- Allow file viewing and downloading without login
- Hide create/edit/delete functions for guests
- Add guest mode banner with login prompt
- Add read-only badges for guest accessible files
- Include permission checks for all CRUD operations
- Add responsive guest mode styling
- Support both online (Supabase) and offline (localStorage) modes

Features:
• Guest users can view all files and download attachments
• Authentication required for create, edit, delete operations
• Seamless transition between guest and authenticated modes
• User-friendly guest experience with clear login prompts

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

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

styles.css

@@ -120,6 +120,50 @@ header p {
     color: #856404;
 }
 
+/* 게스트 모드 스타일 */
+.guest-mode {
+    background: linear-gradient(135deg, #a8e6cf, #88d8a3);
+    color: #2d3436;
+    padding: 15px 20px;
+    border-radius: 8px;
+    margin: 20px 0;
+    border: 2px solid #00b894;
+    box-shadow: 0 2px 10px rgba(0, 184, 148, 0.2);
+}
+
+.guest-mode-content {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    flex-wrap: wrap;
+    gap: 10px;
+}
+
+.guest-login-btn {
+    background: #00b894;
+    color: white;
+    border: none;
+    padding: 8px 16px;
+    border-radius: 5px;
+    cursor: pointer;
+    font-weight: 500;
+    transition: all 0.3s ease;
+}
+
+.guest-login-btn:hover {
+    background: #00a085;
+    transform: translateY(-1px);
+}
+
+.read-only-badge {
+    background: #74b9ff;
+    color: white;
+    padding: 4px 8px;
+    border-radius: 4px;
+    font-size: 0.8em;
+    font-weight: 500;
+}
+
 .sync-status {
     display: inline-flex;
     align-items: center;
@@ -569,4 +613,13 @@ header p {
     .file-actions button {
         width: 100%;
     }
+    
+    .guest-mode-content {
+        flex-direction: column;
+        text-align: center;
+    }
+    
+    .guest-login-btn {
+        margin-top: 10px;
+    }
 }