fix: add missing Instance sub-panel titles and overhaul reset password UX

- Add <h2> title headings to General, Streaming, Storage, Users panels
- Reset password now shows confirmation dialog before executing
- Temp password displays inline below the user row (not at top of list)
- Temp password has copy button and dismiss action

packages/web/src/components/modals/instanceSettingsPanels/GeneralPanel.tsx

@@ -65,6 +65,7 @@ export function GeneralPanel() {
 
   return (
     <form className="space-y-5" onSubmit={(e) => e.preventDefault()}>
+      <h2 className="text-lg font-semibold text-txt-primary">General</h2>
       <div className="text-xs text-txt-tertiary">
         Configure your Backspace instance. These settings affect all users.
       </div>