Initial scaffold: Sinatra map app with Tauri desktop shell

Sinatra backend serving a basic Leaflet map, wrapped in a Tauri
desktop shell that starts the same server in a native window.

docs/development.adoc

@@ -0,0 +1,77 @@
+= Development guide
+:toc:
+:toclevels: 2
+
+== Web app only
+
+[source,shell]
+----
+bundle install
+bundle exec puma -C config/puma.rb
+----
+
+Open http://127.0.0.1:4567. For auto-reload during development, run
+via the `rerun` gem instead:
+
+[source,shell]
+----
+bundle exec rerun -- bundle exec puma -C config/puma.rb
+----
+
+== Desktop shell (Tauri)
+
+=== Prerequisites
+
+* Rust (stable) and the Tauri CLI: `cargo install tauri-cli --version "^2"`
+* Ruby + Bundler, with gems installed (`bundle install` in the project
+  root)
+* Platform build tools required by Tauri itself:
+** *Linux*: WebKitGTK and GTK development packages, e.g. on
+   Debian/Ubuntu:
++
+[source,shell]
+----
+sudo apt install libwebkit2gtk-4.1-dev libjavascriptcoregtk-4.1-dev \
+  libgtk-3-dev libsoup-3.0-dev build-essential
+----
+** *macOS*: Xcode command line tools.
+** *Windows*: the WebView2 runtime (preinstalled on current Windows) and
+   the MSVC build tools.
+
+See the upstream https://tauri.app/start/prerequisites/[Tauri
+prerequisites] page for the current package names on your platform.
+
+=== Running in development
+
+[source,shell]
+----
+cd src-tauri
+cargo tauri dev
+----
+
+This runs `beforeDevCommand` (starts Puma on `127.0.0.1:4567`) and
+opens a native window pointed at it, per `src-tauri/tauri.conf.json`.
+
+=== Building
+
+[source,shell]
+----
+cd src-tauri
+cargo tauri build
+----
+
+Requires `Gemfile.lock` to exist (`bundle install` beforehand), since
+the Ruby app files are embedded into the bundle as resources. See
+`docs/architecture.adoc` for the current limitation around this build
+still depending on a system Ruby installation on the target machine.
+
+=== Regenerating icons
+
+Desktop icons live in `src-tauri/icons/`. To regenerate them from a
+new source image (at least 1024x1024 PNG):
+
+[source,shell]
+----
+cd src-tauri
+cargo tauri icon path/to/source.png
+----