feat(pdf): wire double-tap to toggle zoom in viewer

The zoomable library's onDoubleTap callback is not wired by default,
so double-tapping the PDF page did nothing — no scale change, no
hi-res re-render. Pass an onDoubleTap handler that calls
zoomState.toggleScale(2.5x, tapPosition). The scale animation runs
through the same Animatable snapshotFlow already observes, so the
debounced hi-res render kicks in automatically once the animation
settles.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/components/pdf/PdfViewerDialog.kt

@@ -85,6 +85,7 @@ import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
 import kotlinx.coroutines.withContext
 import net.engawapg.lib.zoomable.rememberZoomState
+import net.engawapg.lib.zoomable.toggleScale
 import net.engawapg.lib.zoomable.zoomable
 
 // Hard ceiling on each base-rendered page bitmap, in pixels. Higher = sharper when
@@ -100,6 +101,10 @@ private const val HI_RES_MAX_DIM_PX = 6144
 private const val HI_RES_ZOOM_THRESHOLD = 1.2f
 private const val HI_RES_DEBOUNCE_MS = 200L
 
+// Zoom level the viewer animates to when the user double-taps. Matches the
+// threshold region where we swap in the hi-res bitmap.
+private const val DOUBLE_TAP_ZOOM_SCALE = 2.5f
+
 // How many recently-rendered pages to keep around. Pager already pre-composes the
 // current page plus one neighbor; this just speeds up small back/forward swipes.
 // At VIEWER_MAX_DIM_PX = 3072 this caps memory at ~80 MB worth of page bitmaps.
@@ -357,7 +362,12 @@ private fun PdfPageView(
                 modifier =
                     Modifier
                         .fillMaxSize()
-                        .zoomable(zoomState),
+                        .zoomable(
+                            zoomState = zoomState,
+                            onDoubleTap = { position ->
+                                zoomState.toggleScale(targetScale = DOUBLE_TAP_ZOOM_SCALE, position = position)
+                            },
+                        ),
             )
         } else {
             CircularProgressIndicator(color = Color.White)