Issue #18: Add page-by-page preview in the GUI after building

Summary

Add a PDF preview panel to the GUI that appears after a successful build. It renders PDF pages as images using Apache PDFBox and displays them with previous/next navigation and a page counter. The preview loads pages lazily for performance and updates automatically on new builds.

AC Verification Checklist

After a successful build, a preview panel appears showing generated pages
Users can navigate between pages (previous/next buttons)
Current page number and total count displayed (e.g., "Seite 3 / 42")
Preview renders actual PDF pages as images (PDF-to-image via PDFBox)
Preview panel can be closed/hidden to return to normal view
Preview updates automatically when a new build completes
GUI remains responsive while preview is loading (async rendering)

Implementation Steps

Step 1: Add PDFBox dependency to gui module

Add org.apache.pdfbox:pdfbox:3.0.4 to gui/build.gradle.kts.

Step 2: Create PdfPreviewState class

A state holder for the preview: current page index, total pages, rendered page images (cached), loading state. Pages are rendered lazily — only the current page is rendered at a time.

Step 3: Create PdfPreviewPanel composable

A Compose panel with:

An Image composable showing the current page
Navigation row: "< Prev" button | "Seite X / Y" label | "Next >" button
A close/hide button
Loading indicator while page is rendering

Step 4: Integrate preview into App composable

After a successful build:

Show a "Vorschau" button in the action buttons row
When clicked, show the preview panel (replacing or overlaying the song list area)
When a new build succeeds, update the preview automatically

Step 5: Lazy page rendering

Render pages on demand using coroutines on Dispatchers.IO to keep the UI responsive.

1.8 KiB Raw Permalink Blame History