docs: add CLAUDE.md for AI-assisted development

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## What this repository is
+
+This is the **super-repo** for Euro-Office Desktop Editors — a rebrand of
+ONLYOFFICE DocumentServer/DesktopEditors (see `ATTRIBUTION`, AGPL-3.0). It is a
+build-orchestration meta-repo: **the actual source code lives in 9 git
+submodules**, all forks under `github.com/Euro-Office`. The super-repo itself
+contains only the Docker Bake build pipeline, branding/config overlays, CI, and
+docs.
+
+The only first-party source tracked here is:
+- `build/docker-bake.hcl` + `build/.docker/*.Dockerfile` — the build orchestration
+- `build/configs/core/DoctRenderer.config.desktop` — converter runtime config
+- `.github/workflows/` — CI
+- `CHANGELOG.md`, `README.md`, `ATTRIBUTION`, `LICENSE`
+
+### Submodules must be initialized first
+A bare checkout has **empty submodule directories** (no source). Before doing
+anything, initialize them:
+```sh
+git submodule update --init --recursive
+```
+The submodule directories are pinned to specific commits via `git submodule
+status`; the super-repo's job is to lock a consistent set of submodule SHAs and
+build them together.
+
+## Build commands
+
+All builds run through Docker Bake from the `build/` directory:
+```sh
+cd build
+docker buildx bake              # default target: desktop-export → ./deploy/desktop
+docker buildx bake packages     # build .deb/.rpm/.tar.xz → ./deploy/packages
+docker buildx bake deps         # build dependency images only (core, sdkjs, web-apps, desktop-js)
+```
+There is no separate compile/lint/test step at the super-repo level — building
+*is* `docker buildx bake`, and per-component tests live inside the submodules.
+CI here (`.github/workflows/check.yml`) only markdown-lints and spell-checks
+`CHANGELOG.md` (custom dictionary: `.aspell.en.pws`); `winget.yml` publishes to
+WinGet on GitHub release.
+
+### Build-time variables (override via env or `--set`)
+Defined as `variable` blocks in `build/docker-bake.hcl`; override with an env var
+of the same name, e.g. `PRODUCT_VERSION=9.4.0 docker buildx bake`:
+- `PRODUCT_VERSION` (e.g. `9.3.1`), `BUILD_NUMBER`
+- `COMPANY_NAME` / `PRODUCT_NAME` — branding strings (`COMPANY_NAME_LOW` is derived)
+- `REGISTRY`, `TAG` — image naming
+- `BRANDING_DIR` (default `.`) — root of a branding overlay tree that mirrors
+  submodule paths; the `packages` stage copies upstream `desktop-apps/package/`
+  then overlays `${BRANDING_DIR}/desktop-apps/package/` on top
+- `NUGET_CACHE` (`local`|`remote`), `CACHE_BUST`
+
+## Build architecture (the Docker Bake DAG)
+
+Understanding the build means reading `docker-bake.hcl` together with the
+`.docker/*.bake.Dockerfile` in each submodule. Each submodule ships its own
+Dockerfile under `<submodule>/.docker/`; `docker-bake.hcl` wires them together
+as named build *contexts* (`contexts = { name = "target:other-target" }`), so
+stages consume each other's outputs without intermediate registries.
+
+Dependency-target images (group `deps`):
+- `core-base`, `core-wasm` ← `core/.docker/` — C++ server core (native + WASM)
+- `desktop-js` ← `desktop-apps/.docker/` — desktop frontend JS / login page
+- `sdkjs-desktop` ← `sdkjs/.docker/` — JS editor SDK; **depends on `core-wasm`**
+- `web-apps` ← `web-apps/.docker/` — document-server web frontend
+
+Assembly:
+- `desktop-builder` ← `desktop-apps/.docker/` — pulls in `core-base`,
+  `desktop-js`, `sdkjs-desktop`, `web-apps`
+- `desktop-export` ← `build/.docker/desktop-composer.bake.Dockerfile` — the
+  **final filesystem layout** is assembled here: copies editors/web-apps/sdkjs,
+  converter binaries + `DoctRenderer.config`, document templates, dictionaries,
+  fonts, then regenerates `AllFonts.js`/`font_selection.bin` via `allfontsgen`
+  and slide themes via `allthemesgen`. Outputs via a `FROM scratch` stage to
+  `./deploy/desktop`.
+- `packages` ← `build/.docker/packages.bake.Dockerfile` — on `ubuntu:24.04`,
+  runs the upstream `desktop-apps/package/` Makefile (`make deb rpm tar`) over
+  the `desktop-export` output. Outputs `.deb`/`.rpm`/`.tar.xz` to `./deploy/packages`.
+
+The runtime entrypoint produced by the composer is `start_desktop.sh`, which
+launches the CEF/Chromium-based `DesktopEditors` binary with
+`LD_PRELOAD=libcef.so`.
+
+## Things that are easy to get wrong
+
+- **Only the Nextcloud cloud provider is currently bundled.** The composer
+  copies `desktop-apps/common/loginpage/providers/nextcloud` only — adding other
+  providers (ownCloud, Seafile, etc.) requires editing
+  `desktop-composer.bake.Dockerfile`.
+- Editing a Dockerfile for a *component* means editing it **in that submodule**,
+  not here. Only the composer and packaging Dockerfiles live in `build/.docker/`.
+- Bumping a component means committing the new submodule SHA in this super-repo
+  (`chore: update submodules`-style commits) so the locked set stays consistent.