Feature/toasts and error handling (#4496)

# Description of Changes

- Added error handling and toast notifications

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/types/fileContext.ts

@@ -219,6 +219,7 @@ export interface FileContextState {
     isProcessing: boolean;
     processingProgress: number;
     hasUnsavedChanges: boolean;
+    errorFileIds: FileId[]; // files that errored during processing
   };
 }
 
@@ -241,6 +242,9 @@ export type FileContextAction =
   | { type: 'SET_SELECTED_PAGES'; payload: { pageNumbers: number[] } }
   | { type: 'CLEAR_SELECTIONS' }
   | { type: 'SET_PROCESSING'; payload: { isProcessing: boolean; progress: number } }
+  | { type: 'MARK_FILE_ERROR'; payload: { fileId: FileId } }
+  | { type: 'CLEAR_FILE_ERROR'; payload: { fileId: FileId } }
+  | { type: 'CLEAR_ALL_FILE_ERRORS' }
 
   // Navigation guard actions (minimal for file-related unsaved changes only)
   | { type: 'SET_UNSAVED_CHANGES'; payload: { hasChanges: boolean } }
@@ -269,6 +273,9 @@ export interface FileContextActions {
   setSelectedFiles: (fileIds: FileId[]) => void;
   setSelectedPages: (pageNumbers: number[]) => void;
   clearSelections: () => void;
+  markFileError: (fileId: FileId) => void;
+  clearFileError: (fileId: FileId) => void;
+  clearAllFileErrors: () => void;
 
   // Processing state - simple flags only
   setProcessing: (isProcessing: boolean, progress?: number) => void;