Improve error handling for unavailable network shares

- Add OSError/PermissionError handling in browse endpoint
- Return 503 status code when folders are temporarily unavailable
- Display user-friendly error messages in the UI
- Enhance error logging with exception type information

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

media_server/static/js/app.js

@@ -1579,7 +1579,14 @@ async function browsePath(folderId, path, offset = 0, nocache = false) {
             { headers: { 'Authorization': `Bearer ${token}` } }
         );
 
-        if (!response.ok) throw new Error('Failed to browse path');
+        if (!response.ok) {
+            let errorMsg = 'Failed to browse path';
+            if (response.status === 503) {
+                const errorData = await response.json().catch(() => ({}));
+                errorMsg = errorData.detail || 'Folder is temporarily unavailable (network share not accessible)';
+            }
+            throw new Error(errorMsg);
+        }
 
         const data = await response.json();
         currentPath = data.current_path;
@@ -1602,7 +1609,8 @@ async function browsePath(folderId, path, offset = 0, nocache = false) {
         saveLastBrowserPath(folderId, currentPath);
     } catch (error) {
         console.error('Error browsing path:', error);
-        showToast(t('browser.error_loading'), 'error');
+        const errorMsg = error.message || t('browser.error_loading');
+        showToast(errorMsg, 'error');
         clearBrowserGrid();
     }
 }