fix(android): keyboard covers text input on chat screen (#70)

The KeyboardAvoidingView used behavior='height' on Android, which
conflicts with the native android:windowSoftInputMode='adjustResize'
set in AndroidManifest.xml. This causes the keyboard to overlap the
text input instead of pushing it up.

Fix: use behavior={undefined} on Android, letting the native
adjustResize handle keyboard avoidance — the recommended approach.

Closes #53

(cherry picked from commit 5d2380b794a240e7ee9b72a95b1b5161403704d2)

Co-authored-by: cloph <128580843+cloph-dsp@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/session/[id].tsx

@@ -483,7 +483,7 @@ export default function SessionScreen() {
 
       <KeyboardAvoidingView
         style={[s.container, isDark && s.containerDark]}
-        behavior={Platform.OS === "ios" ? "padding" : "height"}
+        behavior={Platform.OS === "ios" ? "padding" : undefined}
         keyboardVerticalOffset={Platform.OS === "ios" ? 90 : 0}
       >
         {/* Session info pulldown */}