Replace white airline logo with colored sample icon

Changed from R.drawable.skyairlines (white logo) to R.drawable.sample_icon
(colored icon) so the image is visible in both light and dark modes.

The white logo was invisible on white backgrounds in light mode, which
made the Image slot appear empty. The sample icon has colors that work
in both themes.

Files updated:
- Test and story files to use sample_icon
- Regenerated all Image slot screenshots (docs + test snapshots)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: YogiPatelSkyScanner

app/screenshots/oss/debug/default/net.skyscanner.backpack.compose.cellitem.BpkCellItemTest_withImage.png

@@ -166,7 +166,7 @@ fun CellItemWithImageStory(modifier: Modifier = Modifier) {
             title = "Partner Program",
             body = "Skyland Airlines",
             onClick = {},
-            slot = BpkCellItemSlot.Image(R.drawable.skyairlines),
+            slot = BpkCellItemSlot.Image(R.drawable.sample_icon),
             modifier = Modifier.fillMaxWidth(),
         )
     }

app/screenshots/oss/debug/dm/net.skyscanner.backpack.compose.cellitem.BpkCellItemTest_withImage.png

@@ -104,7 +104,7 @@ class BpkCellItemTest : BpkSnapshotTest() {
             icon = BpkIcon.Account,
             title = "Partner Program",
             body = "Skyland Airlines",
-            slot = BpkCellItemSlot.Image(R.drawable.skyairlines),
+            slot = BpkCellItemSlot.Image(R.drawable.sample_icon),
         )
     }