fix(mobile): pin the chat composer above the keyboard on Android browsers

The chat screen relied on the browser's focused-field reveal, which holds
on iOS Safari but not on Android, where interactive-widget is also widely
ignored — the composer just stayed behind the keyboard. The draft screen's
visual-viewport pin now covers the chat screen on Android; iOS chat keeps
the native reveal.

packages/ui/src/components/chat/composer/state/useMobileViewportPin.ts

@@ -17,6 +17,15 @@ import React from 'react';
 import { isCapacitorApp } from '@/lib/platform';
 import type { ComposerEditorHandle } from '../editor/ComposerEditor';
 
+// Android mobile browsers are the pan-mode holdouts this pin exists for on
+// the CHAT screen too: interactive-widget=resizes-content is ignored by a
+// fair share of Android WebView/Chrome builds, and unlike iOS Safari they do
+// not reliably reveal the focused field either — the composer just stays
+// behind the keyboard. iOS keeps its browser-native reveal on the chat
+// screen, so this stays Android-only there.
+// Callers are browser-only React effects, so navigator always exists here.
+const isAndroidBrowser = (): boolean => /Android/i.test(navigator.userAgent);
+
 export interface MobileViewportPinOptions {
     isMobile: boolean;
     /** Composer expanded to fullscreen on mobile. */
@@ -96,12 +105,14 @@ export function useMobileViewportPin(options: MobileViewportPinOptions): void {
         };
     }, [editorRef, formRef, isFullscreen, isMobile]);
 
-    // Draft screen with the keyboard up: anchor the normal-height composer to
+    // Keyboard up: anchor the normal-height composer to the visible bottom.
-    // the visible bottom. The chat screen does not need this — its own
+    // Draft screen on every mobile browser; chat screen only on Android,
-    // focused-field reveal works there.
+    // where neither viewport resizing nor the focused-field reveal can be
+    // relied on (iOS chat keeps the browser's own reveal).
     React.useLayoutEffect(() => {
         if (!isMobile || isCapacitorApp()) return;
-        if (!isDraftScreen || isFullscreen || !isFocused) return;
+        if (isFullscreen || !isFocused) return;
+        if (!isDraftScreen && !isAndroidBrowser()) return;
         const vv = window.visualViewport;
         const form = formRef.current;
         if (!vv || !form) return;