Add CI

Nothing was gated on a push. .gitea/workflows/ci.yml runs gofmt, build,
vet, test and pnpm check on push and pull_request.

web/dist/.gitkeep is tracked on purpose so `go build ./...` succeeds
without a frontend build, so the Go job needs no Node step. The web job
pins pnpm 12: pnpm/action-setup reads packageManager from a root
package.json that does not exist here, and pnpm 10 rejects the allowBuilds
key in web/pnpm-workspace.yaml.

Not verified against a real runner - the first push is the test.

.gitea/workflows/ci.yml

@@ -0,0 +1,36 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.26'
+
+      - name: gofmt
+        run: test -z "$(gofmt -l .)"
+
+      - run: go build ./...
+
+      - run: go vet ./...
+
+      - run: go test ./...
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+
+      - uses: pnpm/action-setup@v4
+        with:
+          version: '12'
+
+      - name: web check
+        working-directory: web
+        run: pnpm install --frozen-lockfile && pnpm check