feat(desktop): packaging Windows + macOS (C3/C4) : icône, cibles, doc de build

resources/icon.png (source unique, electron-builder dérive tous les formats/tailles par OS). electron-builder.yml : cibles Windows (nsis+portable) et macOS (dmg/zip non signé) déjà présentes. daemon.ts gère SIGTERM puis SIGKILL (Windows sans SIGTERM natif).

README desktop : build par OS (dist:linux/win/mac), ConPTY Windows + SmartScreen (éditeur inconnu), Gatekeeper macOS (clic droit Ouvrir / xattr), limites du cross-build, prérequis git/claude sur le PATH. Les builds Windows/macOS se font sur ces OS.

packages/desktop/README.md

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+# Arboretum Desktop
+
+Native desktop shell (Electron) for Arboretum. It runs the existing daemon as a child process and
+shows its web UI in a window, already authenticated (no login screen). The heavy lifting stays in
+the daemon; this package is a thin shell (window lifecycle, daemon supervision, auto auth).
+
+This package is intentionally **outside the root npm workspaces** so the daemon CI stays light. It
+has its own `package-lock.json` and is built on a developer machine (or a dedicated CI runner),
+not by the main `npm run build`.
+
+## How it works
+
+1. The shell picks a data directory under the OS user-data path and spawns the bundled Node runtime
+   running the packaged daemon (`build/server/package/dist/index.js`) with `ARBORETUM_EMIT_TOKEN_FD=3`.
+2. The daemon mints a fresh token and writes `{token, url}` on file descriptor 3 (private stdio pipe).
+3. The shell posts that token to `/api/v1/auth/login` from the window's session (server to server),
+   which drops the `arb_session` cookie into the session jar, then loads the SPA on `127.0.0.1`.
+4. On quit, the daemon child gets `SIGTERM` (then `SIGKILL` after a grace delay).
+
+A standalone Node runtime (pinned, >= 22.16) is bundled instead of reusing Electron's Node, so
+`node:sqlite` works without a flag and the `node-pty` prebuild keeps the `node.` ABI prefix.
+
+## Prerequisites (all platforms)
+
+- Node >= 22.16 to build.
+- `git` on PATH at runtime (worktree operations). `claude` is discovered on PATH or via the
+  in-app Claude CLI setting; it is not bundled.
+
+## Develop
+
+```bash
+cd packages/desktop
+npm install            # ELECTRON_SKIP_BINARY_DOWNLOAD=1 to skip the Electron binary if you only typecheck
+npm run dev            # bundles main/preload, then `electron .` against the repo's built daemon
+```
+
+`npm run dev` runs the daemon from the repo (`packages/server/dist`, so run `npm run build` at the
+repo root first) using the system `node`.
+
+## Build installers
+
+Each command builds the shell, prepares the daemon (`npm pack` + runtime deps with the right
+`node-pty` prebuild) and a standalone Node runtime, then runs electron-builder.
+
+```bash
+npm run dist:linux     # AppImage + .deb  (on Linux)
+npm run dist:win       # NSIS + portable  (on Windows)
+npm run dist:mac       # dmg + zip        (on macOS)
+```
+
+Artifacts land in `packages/desktop/release/`.
+
+### Linux
+
+Fully supported. `dist:linux` runs on a Linux host or the Gitea CI runner.
+
+### Windows
+
+Build on a Windows host (recommended): the `node-pty` win32 native binary and the installer
+(`makensis`) are most reliable there. Cross-building from Linux via Wine is a best-effort fallback.
+The app uses ConPTY (Windows 10 1809+). The installer is not code-signed yet, so SmartScreen shows
+"unknown publisher": choose "More info" then "Run anyway".
+
+### macOS (best-effort)
+
+Build on a Mac (`dmg`/`zip` cannot be produced elsewhere). The app is **not** signed or notarized,
+so Gatekeeper blocks the first launch: right-click the app then "Open", or run
+`xattr -dr com.apple.quarantine /Applications/Arboretum.app`.
+
+## Auto-update
+
+electron-builder emits `latest*.yml` next to the artifacts; `electron-updater` (wired in a later
+change) points at the Gitea release assets. Auto-update works for Windows (NSIS) and Linux
+(AppImage); macOS updates are manual while the app is unsigned.
+
+## Icon
+
+`resources/icon.png` (square, >= 512px) is the single source; electron-builder derives every
+platform icon from it.