move menu styling to theme for app-wide consistency

Add menuTheme and menuButtonTheme to ThemeData matching the existing
overlay surface style (12px radius, outline border, compact density).
Remove per-use styling from chat_tabs.dart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/theme.dart

@@ -136,6 +136,24 @@ class FrostyThemes {
         backgroundColor: backgroundColor,
         surfaceTintColor: backgroundColor,
       ),
+      menuTheme: MenuThemeData(
+        style: MenuStyle(
+          padding: const WidgetStatePropertyAll(
+            EdgeInsets.symmetric(vertical: 4),
+          ),
+          shape: WidgetStatePropertyAll(
+            RoundedRectangleBorder(
+              borderRadius: const BorderRadius.all(Radius.circular(12)),
+              side: BorderSide(color: borderColor),
+            ),
+          ),
+        ),
+      ),
+      menuButtonTheme: MenuButtonThemeData(
+        style: MenuItemButton.styleFrom(
+          visualDensity: VisualDensity.compact,
+        ),
+      ),
       dialogTheme: DialogThemeData(
         backgroundColor: backgroundColor,
         surfaceTintColor: backgroundColor,