Fix CI: install Cypress binary explicitly before running e2e

Same root cause as the README caveat: pnpm install doesn't reliably
trigger Cypress's postinstall binary download, so `cypress run` failed
in CI with "The cypress npm package is installed, but the Cypress
binary is missing." Add an explicit `cypress install` step, and cache
~/.cache/Cypress keyed on the lockfile so subsequent runs don't
re-download it.

.github/workflows/ci.yml

@@ -37,5 +37,15 @@ jobs:
           node-version: 22
           cache: pnpm
 
+      - name: Cache Cypress binary
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/Cypress
+          key: cypress-${{ runner.os }}-${{ hashFiles('pnpm-lock.yaml') }}
+
       - run: pnpm install --frozen-lockfile
+      # pnpm install doesn't reliably trigger Cypress's postinstall binary
+      # download (see apps/web's cypress caveat in the README) — install it
+      # explicitly so `cypress run` finds it.
+      - run: pnpm --filter web exec cypress install
       - run: pnpm --filter web e2e