Move layer numbering to the ENV

[tom2drum]

Description and Related Issue(s)

Resolves #2554

Makes rollup layer numbering configurable so instances can use L2/L3 (or other pairs) instead of the default L2/L1, as requested in Canny.

• Introduces NEXT_PUBLIC_ROLLUP_LAYER_NUMBER (default 2) and centralizes layer labels in lib/rollups/utils.ts (layerLabels.current / layerLabels.parent).
• Replaces hardcoded "L1"/"L2" across rollup UI (block details, deposits, withdrawals, tx batches, status tags, metadata titles, nav, etc.) with these labels.

Proposed Changes

• Added NEXT_PUBLIC_ROLLUP_LAYER_NUMBER and wired it in rollup config and ENV validator.
• Added lib/rollups/utils.ts with layerLabels and status-formatting helpers; all rollup views now use these labels instead of literal "L1"/"L2".
• Updated docs/ENVS.md (new variable and minor wording: "L1" → "parent", "L2" → "rollup" where generic).

Environment variables

• NEXT_PUBLIC_ROLLUP_LAYER_NUMBER (optional, number, default 2) – Layer number of the rollup. Used to show "L2"/"L1" (or e.g. "L3"/"L2") in the UI. Set to 3 for L3 chains.

Breaking or Incompatible Changes

None. Default remains 2 (L2/L1).

Additional Information

• Validator schema and .env.optimism example updated for the new variable.

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added a feature or functionality that is not privacy-compliant (e.g., tracking, analytics, third-party services), I have disabled it for private mode.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request