docs: build instructions for all platforms + Docker + CI

docs/BUILD.md

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+# Build
+
+Docker (Linux), native (any platform), and CI (multi-platform) build instructions.
+
+## Local builds by host
+
+### Windows (current primary dev environment)
+
+Prerequisites: Rust stable, Node 20, Microsoft C++ Build Tools, WebView2.
+
+```bash
+npm ci
+npm run tauri build -- --bundles msi
+```
+
+Output: `src-tauri/target/release/bundle/msi/`.
+
+### macOS
+
+Prerequisites: Xcode command-line tools, Rust stable, Node 20.
+
+```bash
+npm ci
+npm run tauri build -- --bundles app,dmg
+```
+
+Output: `src-tauri/target/release/bundle/{macos,dmg}/`.
+
+Unsigned: first launch requires right-click → Open to bypass Gatekeeper.
+
+### Linux (native)
+
+Prerequisites: see `Dockerfile` for the full apt-get list (webkit2gtk-4.1, librsvg2, etc.).
+
+```bash
+npm ci
+npm run tauri build -- --bundles deb,appimage,rpm
+```
+
+Output: `src-tauri/target/release/bundle/{deb,appimage,rpm}/`.
+
+### Linux via Docker (works on Windows / macOS too)
+
+```bash
+docker compose build build-linux
+docker compose run --rm build-linux bash -c '
+  npm ci &&
+  npm run tauri build -- --bundles deb,appimage,rpm
+'
+```
+
+Output is mounted back to `src-tauri/target/release/bundle/` on the host.
+
+## CI: GitHub Actions
+
+`.github/workflows/release.yml` runs on `git tag v* && git push origin v*`.
+
+1. `version-check` runs `scripts/check-version.sh` to ensure tag matches `package.json`, `Cargo.toml`, `tauri.conf.json`.
+2. Three parallel jobs build for `ubuntu-latest`, `macos-latest`, `windows-latest` using `tauri-action`.
+3. Artifacts are uploaded to the GitHub Release (auto-draft for non-prerelease tags).
+
+To cut a release:
+
+```bash
+# 1. Bump version in three places
+#    package.json:               "version": "0.7.0"
+#    src-tauri/Cargo.toml:       version = "0.7.0"
+#    src-tauri/tauri.conf.json:  "version": "0.7.0"
+# 2. Commit
+git commit -am "chore(release): bump to v0.7.0"
+# 3. Tag and push
+git tag v0.7.0
+git push origin v0.7.0
+```
+
+## Update checker asset selection
+
+`src-tauri/src/commands.rs::pick_asset_for_current_os` picks the right asset by file extension:
+
+| OS      | Extension  |
+|---------|------------|
+| Windows | `.msi`     |
+| macOS   | `.dmg`     |
+| Linux   | `.AppImage` (preferred; deb / rpm downloadable manually from the release page) |
+
+If the preferred extension is missing, the UI falls back to opening the release page.