Remove Tauri desktop shell

Not going to be used — drop src-tauri/ and all references to it in
the docs. MapTool stays a plain Sinatra web app for now.

docs/architecture.adoc

@@ -4,27 +4,8 @@
 
 == Overview
 
-MapTool is a Sinatra web application. The desktop build does not
-reimplement anything natively — it wraps the same Sinatra app in a
-https://tauri.app[Tauri] window, which is the standard way to ship a
-non-JavaScript backend as a desktop app with Tauri: the Rust shell owns
-the native window and starts the web server as a local process; the
-window then simply loads `http://127.0.0.1:4567` as if it were a
-regular page.
-
-[source]
-----
-+----------------------------+
-| Tauri window (WebView)     |
-|  loads http://127.0.0.1:PORT
-+--------------+-------------+
-               |
-               v
-+----------------------------+
-| Sinatra app (Puma)         |
-|  app.rb / config.ru        |
-+----------------------------+
-----
+MapTool is a Sinatra web application, served locally by Puma and
+opened in a browser.
 
 == Directory layout
 
@@ -51,38 +32,10 @@ regular page.
 | `data/`
 | Local, gitignored data files, e.g. `offline.pmtiles`.
 
-| `src-tauri/`
-| The Tauri (Rust) desktop shell.
-
 | `docs/`
 | Project documentation, in AsciiDoc.
 |===
 
-== Desktop shell (Tauri)
-
-In *development*, `src-tauri/tauri.conf.json` sets `beforeDevCommand`
-to start Puma, and `devUrl` points the window at it. Running the dev
-build (see `docs/development.adoc`) starts both together.
-
-In a *packaged build* there is no `beforeDevCommand` hook, so
-`src-tauri/src/lib.rs` starts the server itself (`bundle exec puma`)
-next to the bundled resources when the app launches, and stops it when
-the window closes.
-
-=== Known limitation: production packaging
-
-The packaged-build server spawn currently assumes a Ruby + Bundler
-installation already present on the machine running the app. That is
-fine for internal or developer builds, but not for a distributable,
-self-contained desktop app.
-
-Shipping a real self-contained build would mean embedding a Ruby
-runtime as a Tauri https://tauri.app/develop/sidecar/[sidecar] binary
-(for example, produced with https://github.com/tebako/tebako[tebako]),
-instead of relying on the system Ruby. That decision — and the
-resulting sidecar build step — is not implemented yet and should be
-revisited before any real distribution build.
-
 == Offline maps
 
 Map data is vector, not raster: the app ships
@@ -160,5 +113,4 @@ terms, the API is free for personal/non-commercial use.
 == Networking
 
 The Sinatra server only binds to `127.0.0.1`. It is not meant to be
-reachable from other machines; the Tauri window talks to it over
-loopback only.
+reachable from other machines.