feat: the rail opens into a sidebar with labels from expanded windows on

Decided by the developer on 9 and 10 Nov 2026: phones keep the bar,
medium windows (600 to 840 dp) the rail alone, and from 840 dp a toggle
at the rail's foot opens it into a sidebar 256 dp wide, as Proton
Drive's: the app named beside its icon, each tab its label beside its
icon, the icons where the rail had them, so nothing moves under the
pointer. Its labels come in one after another, top to bottom, all at once
where animations are off. Open or closed is kept per device, in
halkyon_app's Look (look.json, `sidebar`), which Learn keeps in its
DeviceSettings (`sidebarOpen`), as it does the appearance and language.

NavigationFrame takes sidebarOpen and onSidebarChanged (no toggle without
it), and sidebarSections, what the app lists below the tabs, which Todo's
favourites and projects will be. The closed rail keeps Flutter's widget,
its toggle pinned at the foot (trailingAtBottom); the open one is the
frame's own, as the staggered labels need. Learn, Todo, Train and
Meditate keep the choice.

Whats-New: In wide windows, the button at the foot of the side rail opens it into a sidebar with names, and it stays open.
Whats-New-De: In breiten Fenstern öffnet der Knopf unten an der Seitenleiste sie mit Namen, und sie bleibt offen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/README.md

@@ -28,7 +28,9 @@ LeanCode's lints.
 - **Widgets** (`lib/src/widget/`): `showAdaptiveSheet` with `SheetScaffold`;
   `TopGradient`, `ScreenTint`, `ScreenColor`, `TintFrame`, `FramedScreen`
   and `TabVisibility`; `FlushNavigationBar`, and `NavigationFrame`, every
-  app's frame around its tabs (the rail with the app's icon, the bar flush
+  app's frame around its tabs (the rail with the app's icon, opening from
+  expanded windows on into a sidebar with labels and what the app lists
+  there (`sidebarOpen`, `onSidebarChanged`, `sidebarSections`), the bar flush
   with the foot with a fade above it, its rounded top corners and the
   wave a tap sends over it in the colour of the screen in view, the tab
   chosen in that colour made legible (`readableTone`), without Material's