Update UI_TAG in .env.example

[gk-duplo]

What this changes

How it was verified

• ./run.sh --reset, then built and deployed a sample end to end
• ./scripts/build-extension.sh on the affected sample, then deployed it and created one in the UI
• npm ci && npm run build in the affected frontend/
• ./stop.sh --wipe && ./run.sh from a clean slate
• Ran /duplo-extension and confirmed the guidance still matches reality
• Docs only — no runtime behaviour changed

Output

paste the relevant command output here

Blast radius

• This changes .env.example — image tags or ports here are adopted into every user's
  .env on their next ./run.sh after an upgrade. Say so plainly above.
• This changes scripts/upgrade_dev_kit.sh or scripts/init-project.sh — the adoption path
• This changes a workflow under .github/workflows/
• None of the above

Checks

• No .env, tokens, keys, or customer identifiers in the diff
• No build output committed — dist/, node_modules/, **/bin/, **/obj/, backend/sdk-packages/
• Commits are conventional, imperative, and focused
• If a vendored ng-common-lib tarball was added or bumped, its NOTICE sits beside it