refactor(mobile): Use widgets in Immich asset grid (#7140)

* Refactors to use widgets in immich asset grid

Adds comments

* Fixes asset thumbnail absoluteOffset

* feat(mobile): Uses gradient image placeholders in memory lane and in the asset grid (#7142)

* Uses gradient image placeholders in memory lane and in the asset grid

* Changes to create placeholders in immmich image.thumbnail

* removed unused import

* uses context.isDarkTheme and fixed function signature for allAssetsSelected

mobile/lib/modules/home/ui/asset_grid/thumbnail_placeholder.dart

@@ -0,0 +1,41 @@
+import 'package:flutter/material.dart';
+import 'package:immich_mobile/extensions/build_context_extensions.dart';
+
+class ThumbnailPlaceholder extends StatelessWidget {
+  final EdgeInsets margin;
+  final double width;
+  final double height;
+
+  const ThumbnailPlaceholder({
+    super.key,
+    this.margin = EdgeInsets.zero,
+    this.width = 250,
+    this.height = 250,
+  });
+
+  static const _brightColors = [
+    Color(0xFFF1F3F4),
+    Color(0xFFB4B6B8),
+  ];
+
+  static const _darkColors = [
+    Color(0xFF3B3F42),
+    Color(0xFF2B2F32),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: width,
+      height: height,
+      margin: margin,
+      decoration: BoxDecoration(
+        gradient: LinearGradient(
+          colors: context.isDarkTheme ? _darkColors : _brightColors,
+          begin: Alignment.topCenter,
+          end: Alignment.bottomCenter,
+        ),
+      ),
+    );
+  }
+}