fix(agents): keep a conversation drag-selection out of the composer

Dragging a selection from the last message down past the conversation
highlighted the composer's text field. During such a drag everything outside
the conversation is made unselectable, but WebKit keeps editable text
selectable regardless of user-select, so the selection reached the textarea.
Text fields outside the selection scope now also drop out of hit testing while
the drag lasts.

src/styles/global.css

@@ -50,6 +50,12 @@ html[data-selection-lock] *:not([data-selection-scope], [data-selection-scope] *
   -webkit-user-select: none !important;
   user-select: none !important;
 }
+/* WebKit keeps editable text selectable whatever its user-select, so a drag passing over a
+   text field would extend into it; take fields outside the scope out of hit testing instead. */
+html[data-selection-lock]
+  :is(input, textarea, [contenteditable]):not([data-selection-scope], [data-selection-scope] *) {
+  pointer-events: none !important;
+}
 
 input:not([type="checkbox"], [type="radio"], [type="range"], [type="color"]),
 textarea,