fix(gifPicker): extend header background through the gap before pills

Follow-up to the pills-background fix: the GifPickerHeader used a
bottom margin, leaving a transparent strip between the search input and
the pills row where scrolling GIFs still bled through. Switch to bottom
padding so the gap sits inside the View's background.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/features/gifPicker/components/GifPickerHeader.tsx

@@ -29,7 +29,7 @@ export function GifPickerHeader({
       style={[
         native(a.pt_4xl),
         a.relative,
-        a.mb_md,
+        a.pb_md,
         a.flex_row,
         a.align_center,
         !gtMobile && web(a.gap_md),