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/contexts/file/FileReducer.ts

@@ -21,7 +21,8 @@ export const initialFileContextState: FileContextState = {
     selectedPageNumbers: [],
     isProcessing: false,
     processingProgress: 0,
-    hasUnsavedChanges: false
+    hasUnsavedChanges: false,
+    errorFileIds: []
   }
 };
 
@@ -217,6 +218,30 @@ export function fileContextReducer(state: FileContextState, action: FileContextA
       };
     }
 
+    case 'MARK_FILE_ERROR': {
+      const { fileId } = action.payload;
+      if (state.ui.errorFileIds.includes(fileId)) return state;
+      return {
+        ...state,
+        ui: { ...state.ui, errorFileIds: [...state.ui.errorFileIds, fileId] }
+      };
+    }
+
+    case 'CLEAR_FILE_ERROR': {
+      const { fileId } = action.payload;
+      return {
+        ...state,
+        ui: { ...state.ui, errorFileIds: state.ui.errorFileIds.filter(id => id !== fileId) }
+      };
+    }
+
+    case 'CLEAR_ALL_FILE_ERRORS': {
+      return {
+        ...state,
+        ui: { ...state.ui, errorFileIds: [] }
+      };
+    }
+
     case 'PIN_FILE': {
       const { fileId } = action.payload;
       const newPinnedFiles = new Set(state.pinnedFiles);

frontend/src/contexts/file/fileActions.ts

@@ -558,5 +558,8 @@ export const createFileActions = (dispatch: React.Dispatch<FileContextAction>) =
   setHasUnsavedChanges: (hasChanges: boolean) => dispatch({ type: 'SET_UNSAVED_CHANGES', payload: { hasChanges } }),
   pinFile: (fileId: FileId) => dispatch({ type: 'PIN_FILE', payload: { fileId } }),
   unpinFile: (fileId: FileId) => dispatch({ type: 'UNPIN_FILE', payload: { fileId } }),
-  resetContext: () => dispatch({ type: 'RESET_CONTEXT' })
+  resetContext: () => dispatch({ type: 'RESET_CONTEXT' }),
+  markFileError: (fileId: FileId) => dispatch({ type: 'MARK_FILE_ERROR', payload: { fileId } }),
+  clearFileError: (fileId: FileId) => dispatch({ type: 'CLEAR_FILE_ERROR', payload: { fileId } }),
+  clearAllFileErrors: () => dispatch({ type: 'CLEAR_ALL_FILE_ERRORS' })
 });