ci: add CI workflow, Makefile targets, and fix typing keepalive race (#41)

- Add GitHub Actions CI with parallel Go (build/test -race/vet) and Web UI (pnpm build) jobs
- Add Makefile targets: test, vet, check-web, setup, ci
- Fix data race in typing keepalive: remove nil assignment after close(keepaliveDone)
  so the goroutine can safely read the channel without holding the mutex

Co-Authored-By: Duc Nguyen <me@vanducng.dev>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yaml

@@ -0,0 +1,41 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  go:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache-dependency-path: go.sum
+      - run: go build ./...
+      - run: go test -race ./...
+      - run: go vet ./...
+
+  web:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ui/web
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 10
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+          cache-dependency-path: ui/web/pnpm-lock.yaml
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm build