fix(desktop): pin @assistant-ui/store so the cluster shares one tap

The desktop app is built from source on every install/update
(install.ps1 -> npm ci/install -> tsc -b && vite build). The
@assistant-ui packages share an internal reactivity lib,
@assistant-ui/tap, and only interoperate when they all resolve the
SAME tap version.

@assistant-ui/react@0.12.28 and @assistant-ui/core pin tap@^0.5.x
(which exports only "." and "./react"), but the caret range
react -> store@^0.2.9 floated store up to 0.2.18, which bumped its
tap peer to ^0.9.0 and began importing "@assistant-ui/tap/react-shim"
-- an entry point that only exists in the tap 0.9.x line. With the
hoisted tap stuck on 0.5.x, vite build crashed:

    "./react-shim" is not exported ... from package @assistant-ui/tap

i.e. the opaque "apps/desktop build failed (exit 1)" everyone hit when
updating today.

Pin @assistant-ui/store via root overrides to 0.2.13 -- the last
release that targets tap@^0.5.x -- so react/core/store all agree on the
hoisted tap@0.5.14 again. Verified: tsc -b and vite build both pass.

package.json

@@ -36,7 +36,8 @@
     "agent-browser": "^0.26.0"
   },
   "overrides": {
-    "lodash": "4.18.1"
+    "lodash": "4.18.1",
+    "@assistant-ui/store": "0.2.13"
   },
   "engines": {
     "node": ">=20.0.0"