fix(deps): remove unused postcss preset

[drewstone]

Summary of changes

• Remove the unused root postcss-preset-env development dependency and its exclusive lockfile tree.
• This resolves the Node 20.19+ requirement introduced by merged chore: bump postcss-preset-env from 10.6.1 to 11.5.4 #3327 without changing any PostCSS pipeline. All five PostCSS configs use named plugins and none loads this preset.

Proposed area of change

Root development dependencies and lockfile only; no app source changes.

Associated issue(s)

Follow-up to merged #3327.

Verification

• yarn install --immutable passed on Node 24.20.0 and Node 18.20.8.
• yarn nx run-many --all --target=build --outputStyle=stream passed on GTR: 11 build targets and 5 dependencies.
• yarn nx build leaderboard --skip-nx-cache --outputStyle=static passed on Node 18.20.8, including codegen, typecheck, and the Vite bundle.
• The repository pre-push hook passed lint, format, tests, and build.
• git diff --check passed. No browser behavior changed.

Screen Recording

No visible product change.

Harness Validation (Required for Launch-Flow Impact)

No launch flow changes; wallet harness is not applicable.

[tangletools]

✅ Auto-approved PR — 7b3be29f

Blanket team auto-approval is intentional. The merge gates are CI and codex-p1.
No automated review runs on this PR. This approval rests on the rule above alone.

_{tangletools · auto-approval · reason: blanket_auto_approve · 2026-09-30T00:01:08Z}