[Herklos]

.cursor/install.sh hid failures in a few places. This change makes the script stop on any failing step.

• Remove the || true on corepack enable. If corepack is missing or broken, the install stops at that step instead of failing later in an unrelated one.
• Pin uv to 0.12.23 and install it from the versioned installer URL, which matches the comment that calls uv the pinned Python package manager. The script reinstalls uv when the installed version differs and checks the version after installing.
• Run uv sync --frozen in examples/app/backend, so the committed uv.lock is used as-is and is not rewritten during install.

Refreshing examples/app/backend/uv.lock is a separate change. This PR does not modify any lockfile.

Verification:

• bash -n .cursor/install.sh passes.
• The script was run from a fresh clone of this branch with a clean environment and no uv on PATH. It exited 0, installed uv 0.12.23, ran pnpm install and build, and ran uv sync. git status showed only .cursor/install.sh changed, so no lockfile was rewritten.
• With a corepack stub that exits 1, the script exits 1 at the corepack step.