feat(frontend): Upgrade embedPDF to v2.6.0 and migrate to pdf-lib fork, fix attachment/bookmark panel (#5723)

# Description of Changes

Upgrades embedPDF from v2.5.0 to v2.6.0 and migrates from unmaintained
pdf-lib to @cantoo/pdf-lib fork. Adds defensive error handling for
malformed PDFs and improves bridge lifecycle management.

### Changes

**Dependencies**
- Upgrade all @embedpdf/* packages from ^2.5.0 to ^2.6.0
- Replace pdf-lib with @cantoo/pdf-lib (maintained fork with better
TypeScript support)

**PDF Viewer Infrastructure (attachment/bookmark fix)**
- Add useDocumentReady hook to track document lifecycle across bridges
- Implement defensive bridge cleanup to prevent stale registrations
- Fix race condition in document ready state detection by subscribing to
events before checking state

**Link Extraction (updated to cantoo/pdf-lib)**
- Add graceful error handling for PDFs with invalid catalog structures
- Extract enhanced link metadata (tooltips, colors, border styles,
highlight modes)
- Return empty results instead of throwing on malformed PDFs
- Add validation for link creation (destination page bounds, rect
dimensions, color values)

**Signature Flattening  (updated to cantoo/pdf-lib)**
- Improve SVG embedding with three-tier fallback strategy (native
vector, rasterized PNG, placeholder)
- Add proper Unicode handling for PDF form tooltips via
PDFString.decodeText()
- Extract SVG utilities into cleaner strategy pattern

**Form Field Processing  (updated to cantoo/pdf-lib)**
- Add support for display labels vs export values in dropdown/list
fields per PDF spec 12.7.4.4
- Implement caching for expensive field property lookups
- Add proper handling of malformed /Opt arrays


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## Checklist

### General

- [X] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [X] 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)
- [X] I have performed a self-review of my own code
- [X] 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)

- [X] 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]>

frontend/src/core/contexts/ViewerContext.tsx

@@ -151,7 +151,7 @@ interface ViewerContextType {
   // Bridge registration - internal use by bridges
   registerBridge: <K extends BridgeKey>(
     type: K,
-    ref: BridgeRef<BridgeStateMap[K], BridgeApiMap[K]>
+    ref: BridgeRef<BridgeStateMap[K], BridgeApiMap[K]> | null
   ) => void;
 
   // Save changes function - registered by EmbedPdfViewer
@@ -242,7 +242,7 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
   const registerBridge = useCallback(
     <K extends BridgeKey>(
       type: K,
-      ref: BridgeRef<BridgeStateMap[K], BridgeApiMap[K]>
+      ref: BridgeRef<BridgeStateMap[K], BridgeApiMap[K]> | null
     ) => {
       setBridgeRef(bridgeRefs.current, type, ref);
     },