feat: upgrade xterm.js 5.3 to 6.0 for native DEC 2026 synchronized output

xterm.js 6.0.0 natively handles DEC mode 2026 (synchronized output),
which renders Ink's cursor-up redraws atomically at the parser level.
This eliminates split-frame rendering that caused table header loss
and garbled overlapping text in Claude CLI sessions.

- Migrate from xterm/xterm-addon-* to @xterm/* scoped packages
- Update build.mjs and postinstall.js vendor paths
- Remove old xterm 5.x dependencies

package.json

@@ -51,6 +51,10 @@
     "@fastify/compress": "^8.3.1",
     "@fastify/cookie": "^11.0.2",
     "@fastify/static": "^8.0.0",
+    "@xterm/addon-fit": "^0.11.0",
+    "@xterm/addon-unicode11": "^0.9.0",
+    "@xterm/addon-webgl": "^0.19.0",
+    "@xterm/xterm": "^6.0.0",
     "chalk": "^5.3.0",
     "chokidar": "^3.6.0",
     "commander": "^12.1.0",
@@ -59,10 +63,6 @@
     "qrcode": "^1.5.4",
     "uuid": "^10.0.0",
     "web-push": "^3.6.7",
-    "xterm": "^5.3.0",
-    "xterm-addon-fit": "^0.8.0",
-    "xterm-addon-unicode11": "^0.6.0",
-    "xterm-addon-webgl": "^0.16.0",
     "zod": "^4.3.6"
   },
   "devDependencies": {