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

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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/EmbedPdfViewer.tsx

@@ -20,12 +20,11 @@ import { isStirlingFile, getFormFillFileId } from '@app/types/fileContext';
 import { useViewerRightRailButtons } from '@app/components/viewer/useViewerRightRailButtons';
 import { StampPlacementOverlay } from '@app/components/viewer/StampPlacementOverlay';
 import { RulerOverlay, type PageMeasureScales, type PageScaleInfo, type ViewportScale } from '@app/components/viewer/RulerOverlay';
+import type { PDFDict, PDFNumber } from '@cantoo/pdf-lib';
 import { useWheelZoom } from '@app/hooks/useWheelZoom';
 import { useFormFill } from '@app/tools/formFill/FormFillContext';
 import { FormSaveBar } from '@app/tools/formFill/FormSaveBar';
 
-import type { PDFDict, PDFNumber } from '@cantoo/pdf-lib';
-
 // ─── Measure dictionary extraction ────────────────────────────────────────────
 
 async function extractPageMeasureScales(file: Blob): Promise<PageMeasureScales | null> {
@@ -38,7 +37,7 @@ async function extractPageMeasureScales(file: Blob): Promise<PageMeasureScales |
       if (!(measureObj instanceof PDFDict)) return null;
       const rObj = measureObj.lookup(PDFName.of('R'));
       const ratioLabel = (rObj instanceof PDFString || rObj instanceof PDFHexString)
-        ? rObj.decodeText() : '';
+              ? rObj.decodeText() : '';
       // D = distance array, X = x-axis fallback
       let fmtArray = measureObj.lookup(PDFName.of('D'));
       if (!(fmtArray instanceof PDFArray)) fmtArray = measureObj.lookup(PDFName.of('X'));
@@ -213,7 +212,7 @@ const EmbedPdfViewerContent = ({
   const isFormFillToolActive = (selectedTool as string) === 'formFill';
 
   // Form overlays are shown in BOTH modes:
-  // - Normal viewer: form overlays visible (pdf-lib, frontend-only)
+  // - Normal viewer: form overlays visible (PDFium WASM, frontend-only)
   // - formFill tool: form overlays visible (PDFBox, backend)
   const shouldEnableFormFill = true;
 
@@ -881,7 +880,7 @@ const EmbedPdfViewerContent = ({
   useViewerRightRailButtons(isRulerActive, setIsRulerActive);
 
   // Auto-fetch form fields when a PDF is loaded in the viewer.
-  // In normal viewer mode, this uses pdf-lib (frontend-only).
+  // In normal viewer mode, this uses PDFium WASM (frontend-only).
   // In formFill tool mode, this uses PDFBox (backend).
   const formFillFileIdRef = useRef<string | null>(null);
   const formFillProviderRef = useRef(isFormFillToolActive);