feat(browser): implement browser extension for sending files to DocuElevate

- Add complete browser extension with popup UI and background workers
- Support Chrome, Firefox, Edge, and other Chromium-based browsers
- Include context menu integration for quick file sending
- Add comprehensive documentation for users and administrators
- Update main README and API docs to include browser extension
- Implement secure configuration storage in browser extension storage
- Add SSRF-protected URL upload endpoint integration

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

browser-extension/popup/popup.css

@@ -0,0 +1,200 @@
+* {
+    margin: 0;
+    padding: 0;
+    box-sizing: border-box;
+}
+
+body {
+    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
+    font-size: 14px;
+    line-height: 1.5;
+    color: #333;
+    background-color: #f8f9fa;
+    width: 400px;
+    min-height: 300px;
+}
+
+.container {
+    padding: 16px;
+}
+
+.header {
+    display: flex;
+    align-items: center;
+    gap: 12px;
+    margin-bottom: 20px;
+    padding-bottom: 12px;
+    border-bottom: 2px solid #e9ecef;
+}
+
+.logo {
+    width: 32px;
+    height: 32px;
+}
+
+h1 {
+    font-size: 20px;
+    font-weight: 600;
+    color: #2c3e50;
+}
+
+h2 {
+    font-size: 16px;
+    font-weight: 600;
+    color: #495057;
+    margin-bottom: 12px;
+}
+
+.section {
+    margin-bottom: 16px;
+}
+
+.section.hidden {
+    display: none;
+}
+
+.form-group {
+    margin-bottom: 16px;
+}
+
+label {
+    display: block;
+    margin-bottom: 6px;
+    font-weight: 500;
+    color: #495057;
+}
+
+input[type="url"],
+input[type="text"] {
+    width: 100%;
+    padding: 8px 12px;
+    border: 1px solid #ced4da;
+    border-radius: 4px;
+    font-size: 14px;
+    transition: border-color 0.15s ease-in-out;
+}
+
+input[type="url"]:focus,
+input[type="text"]:focus {
+    outline: none;
+    border-color: #4CAF50;
+    box-shadow: 0 0 0 3px rgba(76, 175, 80, 0.1);
+}
+
+small {
+    display: block;
+    margin-top: 4px;
+    color: #6c757d;
+    font-size: 12px;
+}
+
+.btn {
+    width: 100%;
+    padding: 10px 16px;
+    border: none;
+    border-radius: 4px;
+    font-size: 14px;
+    font-weight: 500;
+    cursor: pointer;
+    transition: all 0.15s ease-in-out;
+}
+
+.btn-primary {
+    background-color: #4CAF50;
+    color: white;
+    margin-bottom: 8px;
+}
+
+.btn-primary:hover {
+    background-color: #45a049;
+}
+
+.btn-primary:active {
+    background-color: #3d8b40;
+}
+
+.btn-primary:disabled {
+    background-color: #95c997;
+    cursor: not-allowed;
+}
+
+.btn-secondary {
+    background-color: #6c757d;
+    color: white;
+}
+
+.btn-secondary:hover {
+    background-color: #5a6268;
+}
+
+.info-box {
+    background-color: #e7f3ff;
+    border: 1px solid #b3d9ff;
+    border-radius: 4px;
+    padding: 12px;
+    margin-bottom: 16px;
+}
+
+.info-box p {
+    margin-bottom: 4px;
+}
+
+.info-box strong {
+    color: #0056b3;
+}
+
+.url-display {
+    word-break: break-all;
+    font-family: 'Monaco', 'Courier New', monospace;
+    font-size: 12px;
+    color: #495057;
+}
+
+#status-message {
+    padding: 12px;
+    border-radius: 4px;
+    margin-top: 12px;
+    font-size: 13px;
+}
+
+#status-message.success {
+    background-color: #d4edda;
+    border: 1px solid #c3e6cb;
+    color: #155724;
+}
+
+#status-message.error {
+    background-color: #f8d7da;
+    border: 1px solid #f5c6cb;
+    color: #721c24;
+}
+
+#status-message.info {
+    background-color: #d1ecf1;
+    border: 1px solid #bee5eb;
+    color: #0c5460;
+}
+
+.loading {
+    position: relative;
+}
+
+.loading::after {
+    content: "";
+    position: absolute;
+    width: 16px;
+    height: 16px;
+    top: 50%;
+    left: 50%;
+    margin-top: -8px;
+    margin-left: -8px;
+    border: 2px solid #f3f3f3;
+    border-top: 2px solid #4CAF50;
+    border-radius: 50%;
+    animation: spin 1s linear infinite;
+}
+
+@keyframes spin {
+    0% { transform: rotate(0deg); }
+    100% { transform: rotate(360deg); }
+}