ci: cache npm downloads in frontend-heavy workflows

[rusackas]

SUMMARY

The pre-commit, E2E (Cypress + Playwright), and experimental-Playwright jobs
run npm ci for the frontend with no download cache — actions/setup-node
was configured without cache:, and the cached-dependencies npm cache is
disabled (# cache-restore npm is commented out in bashlib.sh). So every one
of those jobs re-downloads the full package set from the registry, and npm
install is a large chunk of the per-job setup time that dominates these
workflows.

This enables setup-node's built-in npm cache — the lightweight option:

      - uses: actions/setup-node@… # v6
        with:
          node-version-file: './superset-frontend/.nvmrc'
          cache: 'npm'
          cache-dependency-path: 'superset-frontend/package-lock.json'

It caches the ~/.npm download cache (not node_modules), keyed on the
frontend lockfile. npm ci still does a clean install but skips re-fetching
unchanged tarballs. Deliberately avoiding full-node_modules caching, which is
large and could evict the more valuable instrumented-assets cache under the
repo's 10 GB cache budget.

TESTING INSTRUCTIONS

CI-only. First run on each workflow primes the cache; subsequent runs should
show a cache hit on the "Setup Node.js" step and a faster npm ci.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

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[hainenber]

@actions/setup-node actually enables caching by default in v6 so I don't think there would be behavioral change (hope I'm wrong here). This PR, however, would make that fact more visible for future maintainers :D

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