feat: support folder uploads and sanitize upload paths

- Implement `cleanUploadDisplayName` in the backend to safely sanitize uploaded file paths, preserving directory structures while stripping unsafe characters and preventing path traversal.
- Add folder upload capability in the frontend using the Directory Picker API.
- Implement desktop notifications for completed uploads.

backend/templates/pages/home.html

@@ -25,7 +25,7 @@
             <svg viewBox="0 0 24 24" role="img" focusable="false" aria-hidden="true"><path d="M12 16V4m0 0 4 4m-4-4-4 4M5 20h14" /></svg>
           </span>
           <span class="drop-title">Drop files to upload</span>
-          <span class="drop-copy">or click to browse</span>
+          <span class="drop-copy">or click to browse, paste files, or drop a folder</span>
           <span class="drop-meta">Max file size: {{.Data.MaxUploadSize}} · {{.Data.LimitSummary}}</span>
           <input id="file-input" name="file" type="file" multiple>
         </label>
@@ -77,6 +77,7 @@
         <div class="form-footer">
           <p id="file-summary">Choose one or more files to begin.</p>
           <button class="button button-outline install-pwa-button" type="button" data-install-pwa hidden>Install Warpbox</button>
+          <button class="button button-outline folder-picker-button" type="button" data-folder-picker hidden>Choose folder</button>
           <button class="button button-primary" type="submit">Upload files</button>
           <button class="button button-danger upload-new-button" type="button" id="new-upload" hidden>New upload</button>
         </div>