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

.gitignore

@@ -0,0 +1,13 @@
+/havenllo
+/havenllo.exe
+*.exe
+*-test.exe
+*.db
+*.db-shm
+*.db-wal
+coverage.out
+web/node_modules/
+web/dist/*
+!web/dist/.gitkeep
+.codex-*
+.code-prompt.md