fix: add valtio override to resolve React 19 peer dependency warning

[ygd58]

Problem

Running npm install in new-mini-app-quickstart produces peer dependency warnings:

npm warn ERESOLVE overriding peer dependency
npm warn peer react@"^16.8.0 || ^17.0.0 || ^18.0.0" from use-sync-external-store@1.2.0

Caused by valtio@1.13.2 bundling use-sync-external-store@1.2.0 which does not support React 19.

Fix

Added overrides in package.json to force valtio@^2.0.0 which supports React 19.

Testing

npm install  # no valtio warnings ✅

Closes #69