v0.2.1: Change SHA256 input to file upload field

- Replace SHA256 text input with file upload field for checksum files
- Add readChecksumFile() JavaScript function using FileReader API
- Support .sha256 and .txt checksum file formats
- Add Promise-based async handling for file reading
- Add localized error messages for checksum file validation
- Update translations (de_CH) with new strings

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Admin/VersionAdminController.php

@@ -99,10 +99,10 @@ final class VersionAdminController
                         </td>
                     </tr>
                     <tr id="sha256-hash-row" style="display: none;">
-                        <th><label for="new_file_hash"><?php esc_html_e('SHA256 Hash', 'wc-licensed-product'); ?></label></th>
+                        <th><label for="new_checksum_file"><?php esc_html_e('Checksum File', 'wc-licensed-product'); ?></label></th>
                         <td>
-                            <input type="text" id="new_file_hash" name="new_file_hash" class="large-text" placeholder="<?php esc_attr_e('Enter SHA256 checksum...', 'wc-licensed-product'); ?>" pattern="[a-fA-F0-9]{64}" />
-                            <p class="description"><?php esc_html_e('SHA256 checksum of the uploaded file (optional but recommended for integrity verification).', 'wc-licensed-product'); ?></p>
+                            <input type="file" id="new_checksum_file" name="new_checksum_file" accept=".sha256,.txt" />
+                            <p class="description"><?php esc_html_e('Upload a SHA256 checksum file (.sha256 or .txt) to verify file integrity.', 'wc-licensed-product'); ?></p>
                         </td>
                     </tr>
                     <tr>
@@ -218,6 +218,8 @@ final class VersionAdminController
                 'error' => __('An error occurred. Please try again.', 'wc-licensed-product'),
                 'selectFile' => __('Select Download File', 'wc-licensed-product'),
                 'useThisFile' => __('Use this file', 'wc-licensed-product'),
+                'invalidChecksumFile' => __('Invalid checksum file format. File must contain a 64-character SHA256 hash.', 'wc-licensed-product'),
+                'checksumReadError' => __('Failed to read checksum file.', 'wc-licensed-product'),
             ],
         ]);