Open the bundled manual from Help and from Settings

The packages now carry the manual, but nothing in the application opened
it: the help sheet listed gestures and stopped there.

The help sheet gains a Manual button beside Done, and Settings a Manual
row under About beside the version. Both go through dr_ui::manual, which
finds the installed page through dr_plat::system_data_dirs (the package's
share directory on Linux, the executable's directory on Windows), and a
development build also in the checkout it was compiled from. A copy with
no manual says so on the status line rather than doing nothing.

On the desktop the page goes to the system browser. A section is a URL
fragment, and xdg-open's generic mode and Windows' FileProtocolHandler
both turn a file: URL into a path and drop the fragment, so a section is
opened through a one-line redirect page written to the data directory:
the opener gets a plain path, which every opener keeps, and the browser
follows the redirect to index.html#section itself. The launcher behind
the sign-in's open_in_browser is split out so both share it; the https
check stays with the sign-in.

Android has no path to give a browser: an asset is not a file, a copy in
private storage is unreadable to other apps, a file: URI across apps is
refused, and a content: URI leaves the browser resolving every picture
against the provider. So ManualActivity, a WebView reading
file:///android_asset/manual/index.html straight out of the APK, shows
it, started by class name with the section as an extra. JavaScript is
off, links off the page go to the browser, and the theme is day-night so
the page's own light and dark follow the system. A test checks that the
manifest, the Java class and dr_ui agree on the name and the extra.

Cargo.lock

@@ -1717,6 +1717,7 @@ dependencies = [
  "slint-build",
  "thiserror 2.0.20",
  "tokio",
+ "url",
  "wgpu",
 ]