fix(Giphy): use fixed height for giphy preview to prevent layout shifts (#3095)

## Summary
- Set a fixed height (`--str-chat__gif-height`: 200px) on the giphy
container during the preview/shuffle state
- Prevents layout jumps when shuffling between portrait and landscape
giphies
- Keeps the Shuffle/Send/Cancel action buttons visible without scrolling
- Add fixed dimensions to the giphy image placeholder so the layout is
stable when a giphy fails to load
- Removed stale TODO comment that this change resolves

## Test plan
- [ ] Type `/giphy <query>` to trigger a giphy preview
- [ ] Click Shuffle repeatedly — verify the container height stays
stable and action buttons remain visible
- [ ] Verify both portrait and landscape giphies render properly within
the fixed height via `object-fit: contain`
- [ ] After sending, verify the giphy renders at full dynamic height (no
fixed constraint)
- [ ] Simulate a giphy image load failure — verify the placeholder has
correct dimensions

Author: oliverlaz

src/components/Attachment/styling/Giphy.scss

@@ -6,8 +6,6 @@
 }
 
 .str-chat__message-attachment-giphy {
-  // todo: maybe we do not want to restrict the height
-  //height: var(--str-chat__gif-height);
   position: relative;
   display: flex;
   align-items: center;
@@ -26,6 +24,11 @@
     width: 100%;
     cursor: default;
   }
+
+  .str-chat__image-placeholder {
+    height: var(--str-chat__gif-height);
+    width: var(--str-chat__attachment-max-width);
+  }
 }
 
 .str-chat__giphy-badge {
@@ -50,6 +53,14 @@
 }
 
 .str-chat__message-attachment--giphy--actions {
+  .str-chat__message-attachment-giphy {
+    height: var(--str-chat__gif-height);
+
+    img.str-chat__base-image {
+      height: var(--str-chat__gif-height);
+    }
+  }
+
   .str-chat__visibility-disclaimer {
     display: flex;
     padding: var(--spacing-sm, 12px);