feat(pdf): replace PdfLib with Pdfium for form handling and general rendering tasks (#5899)

# Description of Changes

Improves PDF rendering in the viewer by adding digital signature field
support,
cleaning up overlay rendering, and migrating the contrast tool off
pdf-lib to PDFium WASM.

### Signature Field Overlay
- Added `SignatureFieldOverlay` component that renders digital signature
form fields
- Renders appearance streams when present; shows a fallback badge for
unsigned fields
- Uses PDFium WASM for bitmap extraction

### Overlay Rendering
- Integrated `SignatureFieldOverlay` and `ButtonAppearanceOverlay` into
`LocalEmbedPDF`
- Overlays are now clipped to page boundaries
- Clarified in `EmbedPdfViewer` that frontend overlays use PDFium WASM,
  backend overlays use PDFBox

### Contrast Tool Migration
- Replaced pdf-lib with PDFium WASM in `useAdjustContrastOperation`
- PDF page creation and image embedding now go through PDFium APIs
directly
- Updated bitmap handling and memory management accordingly

### Cleanup
- Fixed import ordering in viewer components
- Removed stale comments in the contrast operation hook

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## 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)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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.

---------

Signed-off-by: Balázs Szücs <[email protected]>
Co-authored-by: Reece Browne <[email protected]>

frontend/src/core/components/viewer/LocalEmbedPDF.tsx

@@ -61,6 +61,8 @@ import { DocumentReadyWrapper } from '@app/components/viewer/DocumentReadyWrappe
 import { ActiveDocumentProvider } from '@app/components/viewer/ActiveDocumentContext';
 import { absoluteWithBasePath } from '@app/constants/app';
 import { FormFieldOverlay } from '@app/tools/formFill/FormFieldOverlay';
+import { ButtonAppearanceOverlay } from '@app/tools/formFill/ButtonAppearanceOverlay';
+import SignatureFieldOverlay from '@app/components/viewer/SignatureFieldOverlay';
 import { CommentsSidebar } from '@app/components/viewer/CommentsSidebar';
 import { CommentAuthorProvider } from '@app/contexts/CommentAuthorContext';
 import { accountService } from '@app/services/accountService';
@@ -770,6 +772,7 @@ export function LocalEmbedPDF({ file, url, fileName, enableAnnotations = false,
                             width,
                             height,
                             position: 'relative',
+                            overflow: 'hidden', // clip overlays (buttons, fields) that extend beyond the page rect
                             userSelect: 'none',
                             WebkitUserSelect: 'none',
                             MozUserSelect: 'none',
@@ -790,6 +793,16 @@ export function LocalEmbedPDF({ file, url, fileName, enableAnnotations = false,
                           </div>
                           <TextSelectionHandler documentId={documentId} pageIndex={pageIndex} />
 
+                          {/* ButtonAppearanceOverlay — renders PDF-native button visuals as bitmaps */}
+                          {enableFormFill && file && (
+                            <ButtonAppearanceOverlay
+                              pageIndex={pageIndex}
+                              pdfSource={file}
+                              pageWidth={width}
+                              pageHeight={height}
+                            />
+                          )}
+
                           {/* FormFieldOverlay for interactive form filling */}
                           {enableFormFill && (
                             <FormFieldOverlay
@@ -801,6 +814,17 @@ export function LocalEmbedPDF({ file, url, fileName, enableAnnotations = false,
                             />
                           )}
 
+                          {/* SignatureFieldOverlay — bitmaps of digital-signature appearances */}
+                          {file && (
+                            <SignatureFieldOverlay
+                              documentId={documentId}
+                              pageIndex={pageIndex}
+                              pdfSource={file}
+                              pageWidth={width}
+                              pageHeight={height}
+                            />
+                          )}
+
                           {/* AnnotationLayer for annotation editing and annotation-based redactions */}
                           {(enableAnnotations || enableRedaction) && (
                             <AnnotationLayer