Release 3.2: Nimbus look and feel, code cleanup, automated releases

[abdonmorales]

Summary

Prepares version 3.2 of PDF Compiler.

Codebase improvements (src/PDFCompiler.java)

• Nimbus is now the default look and feel — set at startup with a graceful fallback to the default cross-platform L&F if Nimbus is unavailable.
• Replaced anonymous ActionListener/AboutHandler/DropTargetListener classes with lambdas and DropTargetAdapter.
• Extracted a shared addPdfFile(File) helper used by both the file chooser and drag-and-drop (removes duplicated logic).
• Case-insensitive .pdf matching (accepts .PDF).
• Consolidated merge error handling into a single try/catch (IOException) and dropped the now-unnecessary throws clause.
• Added serialVersionUID; removed unused imports.
• Bumped version to 3.2 in the window title, Javadoc, and About dialog.

Automated releases (.github/workflows/)

• build.yml — CI that compiles on pushes/PRs to main/development.
• release.yml — on a v* tag (or manual dispatch), compiles and bundles a self-contained runnable JAR (PDF Compiler + Apache PDFBox) and publishes it as a GitHub Release asset with auto-generated notes.
• Stopped ignoring .github so the workflows are tracked.

Docs

• README updated with a Release 3.2 section and version bump.

Testing

• javac -Xlint:all compiles cleanly against libs/pdfbox-app-3.0.2.jar (only inherent Swing this-escape constructor warnings remain).
• Verified the fat-JAR build locally: correct Main-Class, bundled PDFCompiler.class, and PDFBox classes present.

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