fix(build): target ES2021 so esbuild keeps xterm 6's requestMode variable

Vite's es2020 default made esbuild 0.25 lower `let r; …(r||={})` to
`void 0||(i={})` without the declaration, so every DECRQM query tmux sends
threw "i is not defined" inside the terminal's parser. Found by a headless
smoke run of the preview build.

client/vite.config.ts

@@ -99,6 +99,12 @@ export default defineConfig({
   },
   build: {
     outDir: "dist",
+    // ES2021 or newer, never Vite's es2020 default: esbuild 0.25 lowers
+    // xterm 6's `let r; …(r||={})` (requestMode, DECRQM) to `void 0||(i={})`
+    // and drops the `let`, so every DECRQM query tmux sends threw
+    // "i is not defined" inside the terminal's parser. React 19 needs these
+    // browsers anyway.
+    target: ["es2021", "chrome100", "edge100", "firefox100", "safari15"],
     // Maps are still written (for debugging a build), but no bundle points at
     // them, so a browser never downloads them.
     sourcemap: "hidden",