feat: implement Havenllo single-board kanban (Go+SQLite backend, Preact/Vite frontend)

- Single fixed 'Haven' board: lists (To do/In progress/Done) + cards, no multi-board
- Go 1.24 backend, CGO-free modernc.org/sqlite, embedded SPA, REST /api
- Preact + Vite + TS frontend, native HTML5 drag-and-drop, optimistic UI, dark Haven theme
- PVC (nfs-client) for SQLite, root container on NFS, simple nginx ingress havenllo.haven
- Dockerfile multi-arch build, Gitea CI via pipeline-actions@main

Dockerfile

@@ -0,0 +1,24 @@
+# syntax=docker/dockerfile:1
+FROM --platform=$BUILDPLATFORM node:22-alpine AS frontend
+WORKDIR /src/web
+COPY web/package.json web/package-lock.json ./
+RUN npm ci
+COPY web/ ./
+RUN npm run build
+
+FROM --platform=$BUILDPLATFORM golang:1.24-alpine AS build
+ARG TARGETOS
+ARG TARGETARCH
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+COPY --from=frontend /src/web/dist ./web/dist
+RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH go build -trimpath -ldflags="-s -w" -o /out/havenllo ./cmd/havenllo
+
+FROM gcr.io/distroless/static-debian12
+USER 0
+COPY --from=build /out/havenllo /havenllo
+EXPOSE 8080
+ENTRYPOINT ["/havenllo"]
+