fix(web): keep the new-session dialog inside its own border box

The dialog shell had a fixed height with no overflow clipping, and its body only scrolled below 640px. On desktop the body overflowed, pushing the actions row and footer outside the shell, so the bottom border drew across the Cancel button.

Author: elkaix

apps/pythinker-web/src/components/NewSessionDialog.vue

@@ -166,6 +166,7 @@ onUnmounted(() => document.removeEventListener('keydown', handleKeydown));
   border: 1px solid var(--line);
   border-top: 2px solid var(--blue);
   border-radius: 4px;
+  overflow: hidden;
   width: 520px;
   max-width: calc(100vw - 32px);
   height: 360px;
@@ -210,6 +211,8 @@ onUnmounted(() => document.removeEventListener('keydown', handleKeydown));
   flex-direction: column;
   gap: 12px;
   flex: 1;
+  overflow-y: auto;
+  min-height: 0;
 }
 
 .form-row {
@@ -349,7 +352,6 @@ onUnmounted(() => document.removeEventListener('keydown', handleKeydown));
     max-height: calc(100dvh - 24px);
   }
   .form-body {
-    overflow-y: auto;
     -webkit-overflow-scrolling: touch;
   }
   .form-row {