refactor(frontend): remove unused React default imports (#4529)

## Description of Changes

- Removed unused `React` default imports across multiple frontend
components.
- Updated imports to only include required React hooks and types (e.g.,
`useState`, `useEffect`, `Suspense`, `createContext`).
- Ensured consistency with React 17+ JSX transform, where default
`React` import is no longer required.
- This cleanup reduces bundle size slightly and aligns code with modern
React best practices.

---

## 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.

Co-authored-by: Reece Browne <[email protected]>

frontend/src/contexts/FileContext.tsx

@@ -12,7 +12,7 @@
  * Memory management handled by FileLifecycleManager (PDF.js cleanup, blob URL revocation).
  */
 
-import React, { useReducer, useCallback, useEffect, useRef, useMemo } from 'react';
+import { useReducer, useCallback, useEffect, useRef, useMemo } from 'react';
 import {
   FileContextProviderProps,
   FileContextSelectors,

frontend/src/contexts/SidebarContext.tsx

@@ -1,4 +1,4 @@
-import React, { createContext, useContext, useState, useRef, useMemo } from 'react';
+import { createContext, useContext, useState, useRef, useMemo } from 'react';
 import { SidebarState, SidebarRefs, SidebarContextValue, SidebarProviderProps } from '../types/sidebar';
 
 const SidebarContext = createContext<SidebarContextValue | undefined>(undefined);
@@ -7,7 +7,7 @@ export function SidebarProvider({ children }: SidebarProviderProps) {
   // All sidebar state management
   const quickAccessRef = useRef<HTMLDivElement>(null);
   const toolPanelRef = useRef<HTMLDivElement>(null);
-  
+
   const [sidebarsVisible, setSidebarsVisible] = useState(true);
   const [leftPanelView, setLeftPanelView] = useState<'toolPicker' | 'toolContent'>('toolPicker');
   const [readerMode, setReaderMode] = useState(false);
@@ -44,4 +44,4 @@ export function useSidebarContext(): SidebarContextValue {
     throw new Error('useSidebarContext must be used within a SidebarProvider');
   }
   return context;
-} 
+}