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/styles/index.css

@@ -18,6 +18,19 @@ code {
     monospace;
 }
 
+/* Suppress font visibility warnings for PDF.js embedded fonts */
+@font-face {
+  font-family: 'Adwaita Mono';
+  src: local('Adwaita Mono');
+  font-display: optional;
+}
+
+@font-face {
+  font-family: 'Nimbus Mono PS';
+  src: local('Nimbus Mono PS');
+  font-display: optional;
+}
+
 /* CSS Variables */
 :root {
   --footer-height: 2rem;