perf: add memCacheWidth/memCacheHeight to all thumbnail images

All CachedNetworkImage instances in list/card contexts now decode at
2x rendered size instead of full resolution. A 3000x2000 event photo
previously decoded to ~24MB in GPU memory even when shown at 96px —
now decodes to <1MB.

Affected screens (16 CachedNetworkImage instances total):
- home_screen.dart: hero (800w), top card (300w), stacked (192w),
  horizontal (440x360), full-width (800x400), search (112x112),
  filter sheet (160x160), type icons (112x112)
- home_desktop_screen.dart: mini (128x128), grid (600x280), horiz (600x296)
- calendar_screen.dart: event card (400x300)
- profile_screen.dart: avatar (size*2), event tile (120x120)

learn_more_screen.dart intentionally unchanged — full-res for detail view.

Estimated memory reduction: ~500MB → ~30MB for a typical home screen.

lib/screens/home_desktop_screen.dart

@@ -777,6 +777,8 @@ class _HomeDesktopScreenState extends State<HomeDesktopScreen> with SingleTicker
           child: img != null
               ? CachedNetworkImage(
                   imageUrl: img,
+                  memCacheWidth: 128,
+                  memCacheHeight: 128,
                   width: 64,
                   height: 64,
                   fit: BoxFit.cover,
@@ -833,6 +835,8 @@ class _HomeDesktopScreenState extends State<HomeDesktopScreen> with SingleTicker
               child: img != null
                   ? CachedNetworkImage(
                       imageUrl: img,
+                      memCacheWidth: 600,
+                      memCacheHeight: 280,
                       width: double.infinity,
                       height: imageHeight,
                       fit: BoxFit.cover,
@@ -944,6 +948,8 @@ class _HomeDesktopScreenState extends State<HomeDesktopScreen> with SingleTicker
               child: img != null
                   ? CachedNetworkImage(
                       imageUrl: img,
+                      memCacheWidth: 600,
+                      memCacheHeight: 296,
                       width: width,
                       height: imageHeight,
                       fit: BoxFit.cover,