[Feature]: CI workflow for lint and production build

[rosspeili]

One-line summary

Add GitHub Actions to run npm run lint and npm run build on avatar-demo for PRs and main.

Problem / motivation

Roadmap item. Label sync CI exists, but regressions in renderer/Electron packaging inputs can land without automated checks. Contributors need a clear green/red signal before merge.

Proposed solution

• Workflow .github/workflows/ci.yml on pull_request + push to main.
• Node 20, npm ci in avatar-demo/, then npm run lint and npm run build.
• Optional: cache npm; do not run dist:win on every PR (slow / Windows-specific) — separate optional workflow later.
• Badge on README once stable (optional follow-up).

Alternatives considered

• Lint-only — better than nothing; build catches Vite/asset graph breaks.
• Full dist:win on CI — expensive; defer.

Primary surface

Other

Constraints you accept

• Should stay usable offline / local-first
• Browser-only mode may remain limited vs Electron

Mock / sketch / reference (optional)

Mirror pattern from other ARPAHLS repos if present; keep YAML minimal.

[AUDOSt0ck1ng]

Split it into 3 PRs so they're easier to review separately:

#34 goes first, it's a prerequisite. Lint was only actually covering src/ — the flat config had one block matching **/.{js,jsx}, so all of electron/.cjs was getting parsed with no rules applied, and scripts/*.mjs wasn't matched at all. Lint was also red on main, so it couldn't be used as a gate yet. Widening it turned up a few real things, the one worth mentioning being VrmAvatar reading group.current in effect cleanup, which means a model swap can detach from a different group than the one it attached to.

#35 is the actual workflow. pull_request + push to main, npm ci then lint, test and build, npm cache on, no dist:win. Runs in about 27s. It's green on my fork if you want to see a real run: https://github.com/AUDOSt0ck1ng/avatar/actions/runs/31142107649

Few things I did differently from the issue, say the word if you'd rather I match it exactly:

• Node 22 rather than 20. Vite 7 wants ^20.19.0 || >=22.12.0, and plain 20 would resolve to a recent enough 20.x so it'd work fine, I just went with the current LTS for a bit more headroom. One line to change.
• avatar/ rather than avatar-demo/, since the rename in 5531413.
• I threw npm test in as well. The existing electron suites finish in 0.26s so it's basically free, and it covers the VRoid Hub auth/client paths. Easy to pull back out.

#36 is the README badge, the optional one. Worth holding until #35 is in, otherwise shields has no workflow to read and it just renders a grey "no status". I used the shields endpoint instead of GitHub's own badge.svg so it sits properly alongside the flat-square badges already in that row, the downside being the colour is status-driven green/red rather than matching the pastels. Happy to swap it for the native one if you'd prefer not to mix styles.