Add offline translation caching for seamless offline experience

- Implemented TranslationCache class with IndexedDB storage
- Cache translations automatically with 30-day expiration
- Added cache management UI in settings modal
  - Shows cache count and size
  - Toggle to enable/disable caching
  - Clear cache button
- Check cache first before API calls (when enabled)
- Automatic cleanup when reaching 1000 entries limit
- Show "(cached)" indicator for cached translations
- Works completely offline after translations are cached

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

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

templates/index.html

@@ -286,6 +286,29 @@
                                 Error notifications
                             </label>
                         </div>
+                        
+                        <hr>
+                        
+                        <h6>Offline Cache</h6>
+                        <div class="mb-3">
+                            <div class="d-flex justify-content-between align-items-center mb-2">
+                                <span>Cached translations:</span>
+                                <span id="cacheCount" class="badge bg-primary">0</span>
+                            </div>
+                            <div class="d-flex justify-content-between align-items-center mb-2">
+                                <span>Cache size:</span>
+                                <span id="cacheSize" class="badge bg-secondary">0 KB</span>
+                            </div>
+                            <div class="form-check form-switch mb-2">
+                                <input class="form-check-input" type="checkbox" id="offlineMode" checked>
+                                <label class="form-check-label" for="offlineMode">
+                                    Enable offline caching
+                                </label>
+                            </div>
+                            <button type="button" class="btn btn-sm btn-outline-danger" id="clearCache">
+                                <i class="fas fa-trash"></i> Clear Cache
+                            </button>
+                        </div>
                     </div>
                     <div class="modal-footer">
                         <div id="settingsSaveStatus" class="text-success me-auto" style="display: none;">