# 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

1. After a successful build, a preview panel appears showing generated pages
2. Users can navigate between pages (previous/next buttons)
3. Current page number and total count displayed (e.g., "Seite 3 / 42")
4. Preview renders actual PDF pages as images (PDF-to-image via PDFBox)
5. Preview panel can be closed/hidden to return to normal view
6. Preview updates automatically when a new build completes
7. 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.