fix(seer): Prevent horizontal scrollbar flash on block entry animation (#112823)

## Changes

The Seer Explorer panel showed a brief horizontal scrollbar flash each
time a new response block loaded.

**Root cause:** The block entry animation in `blockComponents.tsx` uses
`initial={{opacity: 0, x: 10}}`, starting each new block 10px to the
right of its natural position. `BlocksContainer` has `overflow-y: auto`
but no explicit `overflow-x` — CSS coerces `overflow-x` to `auto` when
`overflow-y` is `auto`, so those first few animation frames (while the
block is at `translateX(10px)`) triggered a visible horizontal scrollbar
before the animation resolved to `x: 0`.

**Fix:** Add `overflow-x: hidden` to `BlocksContainer` in
`panelContainers.tsx`. This clips the animation without affecting
vertical scrolling.

Author: aliu39

static/app/views/seerExplorer/panelContainers.tsx

@@ -211,6 +211,7 @@ const PanelContent = styled('div')`
 export const BlocksContainer = styled(Stack)`
   flex: 1;
   overflow-y: auto;
+  overflow-x: hidden;
   overscroll-behavior: contain;
 `;