feat: add page-by-page preview in the GUI after building (Closes #18)

Add a PDF preview panel using Apache PDFBox that appears automatically
after a successful build. Users can navigate pages with prev/next buttons,
see the current page count, and toggle the preview on/off. Pages are
rendered lazily on IO threads to keep the GUI responsive.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gui/build.gradle.kts

@@ -11,6 +11,7 @@ dependencies {
     implementation(compose.desktop.currentOs)
     implementation("io.github.microutils:kotlin-logging-jvm:3.0.5")
     implementation("ch.qos.logback:logback-classic:1.5.16")
+    implementation("org.apache.pdfbox:pdfbox:3.0.4")
 }
 
 compose.desktop {