fix(stream): keep previous items visible while refetching on locale switch

Root cause of the "Bahasa Melayu lags then shows" report: switching
locale invalidates the cache key in usePostStream (streamKey includes
lang), the effect that handles dep changes calls `setItems([])` before
`fetchPage(true)`, and that blanks the entire stream for the duration
of the network round-trip (~150-300ms in dev). The earlier fixes
(AnimatePresence key, font stack, useLayoutEffect title sync) addressed
visible side effects, but this blank window is the actual source of
the perceived freeze.

Drop the eager reset. fetchPage(true) replaces items wholesale once the
new locale's response arrives, so leaving the previous list in place is
a stale-while-revalidate swap: visible content during the gap, single
replace at the end, no skeleton flash, no scroll jump.

Verified in browser with a cn -> ms switch: mainTextLen never hits
zero during the transition, and the new list takes over at t~120ms
without an intervening blank frame.

src/components/messageStream/hooks/usePostStream.ts

@@ -288,7 +288,11 @@ export function usePostStream(params: PostStreamParams): PostStreamResult {
       return;
     }
     restoredFromCacheRef.current = false;
-    setItems([]);
+    // Stale-while-revalidate: keep showing whatever items were on screen for
+    // the previous params (e.g. previous locale) while the new fetch runs.
+    // Clearing to [] here was the root cause of the language-switch flicker
+    // — the stream went blank between the click and the network response.
+    // fetchPage(true) below will replace items wholesale once data arrives.
     cursorRef.current = undefined;
     setHasMore(true);
     hasMoreRef.current = true;