feat(viewer): show a loading spinner while the zoomed original loads

Show a spinner in the top-right of the photo viewer while the original is
being loaded on zoom, so the wait is visible. Only the original triggers it;
the fitted preview and a re-shown preview never do.

Cover the visibility gate with a unit test.

OpenSpec change: add-mobile-photo-zoom

android/app/src/main/java/com/galo/android/ui/viewer/ZoomableImage.kt

@@ -9,6 +9,9 @@ import androidx.compose.foundation.gestures.calculateZoom
 import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
@@ -16,8 +19,10 @@ import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.layout.ContentScale
@@ -25,12 +30,14 @@ import androidx.compose.ui.layout.onSizeChanged
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.platform.testTag
 import androidx.compose.ui.unit.IntSize
+import androidx.compose.ui.unit.dp
 import coil.compose.AsyncImage
 import coil.request.ImageRequest
 import com.galo.android.data.image.ObjectCacheKey
 import com.galo.android.domain.viewer.PhotoZoom
 
 const val TAG_ZOOMABLE_IMAGE = "zoomable_image"
+const val TAG_ZOOM_LOADING = "zoom_loading"
 
 /**
  * Image source shown for the current zoom state: the original when the image is
@@ -61,6 +68,18 @@ internal fun zoomImageRequest(context: Context, source: String?, previewUrl: Str
         .build()
 }
 
+/**
+ * True while the zoomed-in original is loading, i.e. the corner spinner is shown.
+ * Requires a source distinct from the fitted preview, so re-loading the preview
+ * itself (e.g. when no separate original exists) never shows the spinner.
+ */
+internal fun showZoomLoading(
+    zoomed: Boolean,
+    source: String?,
+    previewUrl: String?,
+    loading: Boolean,
+): Boolean = zoomed && loading && source != null && source != previewUrl
+
 /**
  * Photo that can be zoomed and panned, mirroring the web viewer semantics (see
  * `PhotoZoom`). [previewUrl] is shown fitted; once zoomed the image switches to
@@ -88,6 +107,7 @@ fun ZoomableImage(
     var panX by remember { mutableFloatStateOf(0f) }
     var panY by remember { mutableFloatStateOf(0f) }
     var zoomed by remember { mutableStateOf(false) }
+    var originalLoading by remember { mutableStateOf(false) }
 
     fun sync() {
         layerScale = zoom.layerScale
@@ -172,7 +192,9 @@ fun ZoomableImage(
             model = request,
             contentDescription = contentDescription,
             contentScale = ContentScale.Fit,
+            onLoading = { originalLoading = true },
             onSuccess = { state ->
+                originalLoading = false
                 if (natural.width <= 0 || natural.height <= 0) {
                     val drawable = state.result.drawable
                     if (drawable.intrinsicWidth > 0 && drawable.intrinsicHeight > 0) {
@@ -180,6 +202,7 @@ fun ZoomableImage(
                     }
                 }
             },
+            onError = { originalLoading = false },
             modifier = Modifier
                 .fillMaxSize()
                 .graphicsLayer {
@@ -189,5 +212,16 @@ fun ZoomableImage(
                     translationY = panY
                 },
         )
+        if (showZoomLoading(zoomed, source, previewUrl, originalLoading)) {
+            CircularProgressIndicator(
+                color = Color.White,
+                strokeWidth = 2.dp,
+                modifier = Modifier
+                    .align(Alignment.TopEnd)
+                    .padding(16.dp)
+                    .size(24.dp)
+                    .testTag(TAG_ZOOM_LOADING),
+            )
+        }
     }
 }