feat(ci): path-filter gate + concurrency cancel-in-progress for size-limit

[jr200]

Summary

Two wins to cut CI minutes spent on the size-limit job.

1. Path-filter gate

New changes job uses dorny/paths-filter@v3 to detect whether bundle
inputs actually changed in the PR:

• src/**
• public/**
• package.json
• pnpm-lock.yaml
• next.config.*
• .size-limit.json

The size-limit job now depends on both unit and changes, and only
runs when the filter reports changes. Most docs/CI/Makefile PRs skip
the full ~2-minute size-limit job (which runs pnpm build twice —
once on the base branch, once on the PR).

Example: https://github.com/whengas/whengas-ui/pull/38 (Makefile-only)
would have skipped size-limit under this PR, saving the ~2 minutes.

2. Concurrency cancel-in-progress

Added workflow-level concurrency group:

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: ${{ github.event_name == 'pull_request' }}

Only PRs get cancelled — master/tag pushes run to completion so
version-tag verification isn't aborted mid-run.

Caller impact

None — no new inputs, no breaking changes. Callers pick it up
automatically via @master.

Test plan

• YAML valid — same schema plus one new job
• Next PR on whengas-ui with no src changes: changes runs,
  reports bundle=false, size-limit job skipped
• Next PR on whengas-ui touching src/**: changes reports
  bundle=true, size-limit job runs as before
• Force-push to an open PR: older run cancels, new run starts