impl migration to pdfium for split (#6410)

## Summary

Migrates `SplitPDFController`, `SplitPdfByChaptersController`,
`SplitPdfBySizeController` from PDFBox to JPDFium.
`SplitPdfBySectionsController` and `AutoSplitPdfController` are
intentionally left on PDFBox (require JPDFium 1.0.2 features that don't
exist yet).

## Benchmark (audited on `audit/jpdfium-split`, file
`app/core/src/test/java/stirling/software/SPDF/bench/SplitBenchmark.java`)

| Workload | PDFBox heap | JPDFium heap | PDFBox wall | JPDFium wall |
|---|---|---|---|---|
| 100 pp, chunk 10 | +21-26 MB | **+0.02 MB** | 80-106 ms | **25 ms** |
| 300 pp, chunk 10 | +59 MB | **+1.0 MB** | 232 ms | **76 ms** |

**98-99.9% heap reduction. 3-4.2x faster wall.**

## Hybrid

- AcroForm-bearing splits keep PDFBox
`FormUtils.pruneOrphanedFormFields` post-pass (FPDF_ImportPagesByIndex
drops AcroForm dict). Sub-bench shows +1.0 MB / +27 ms - tightly
bounded.
- Metadata extraction stays on PDFBox.
- `SplitPdfBySectionsController` - JPDFium `PdfPageSplitter` only does
2-up halving, not arbitrary MxN.
- `AutoSplitPdfController` - needs PDFRenderer + zxing for QR markers.

## Test plan

- [ ] 30/30 unit tests pass with PDFBox `Loader.loadPDF` as the oracle
(assert page counts + document totals)
- [ ] Existing cucumber feature `split.feature` continues to pass
- [ ] AcroForm-bearing PDF round-trips without orphaned widgets (covered
by existing FormUtils tests)

.dockerignore

@@ -34,8 +34,8 @@ frontend/playwright-report/
 # Tauri/desktop builds
 src-tauri/target/
 src-tauri/dist/
-frontend/src-tauri/target/
-frontend/src-tauri/dist/
+frontend/editor/src-tauri/target/
+frontend/editor/src-tauri/dist/
 
 # IDE and editor
 .idea/