Restructure mobile Settings into a two-level category menu

On mobile, AccountView used to render all 9 Settings sections inline
in one long, individually-collapsible column (_MobileList). Replace
that with a native-style menu: the account card stays pinned at the
top, and every other section becomes a NooSettingsRow in a
NooGroupedList that pushes a dedicated screen (NooTopBar +
NooTopBarBack) holding just that section's content. This removes the
scroll-depth problem outright instead of working around it with
per-section collapsing, so SettingsSection no longer needs
NooGroupedList's collapsible mode on mobile. Desktop's 2-column grid
is unchanged - it already shows every section at once.

Audited every lib/widgets/settings/*.dart file for the reported
"description and controls are flipped" row-layout bug: every row goes
through NooSettingsRow directly, or - for the few hand-rolled rows
(_SavedAccountRow, _ThemeRow/_BottomBarStyleRow, _CacheIntervalRow) -
preserves its icon/description-then-control order (the stacked
control-below-label shape used by _ThemeRow and _CacheIntervalRow is
an intentional, spec'd variant, not a flip). Found no instance of the
bug; no fix was needed.

Updates DESIGN_SYSTEM.md's Settings recipe and styling.md's
NooGroupedList notes to describe the new menu-then-pushed-screen
pattern, and adds a widget test covering the category menu and the
push/pop navigation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/widgets/settings/settings_section.dart

@@ -5,7 +5,7 @@ import '../noo/noo_layout.dart';
 import '../noo/overlays/noo_dialog.dart';
 import '../noo/overlays/noo_sheet.dart';
 
-/// One block of Settings (DESIGN_SYSTEM.md 4's 8-part order): a
+/// One block of Settings (DESIGN_SYSTEM.md 4's 9-part order): a
 /// [NooGroupedList] on mobile (label above a radius-20 card), or a titled,
 /// bordered radius-20 card holding a flat row group on desktop
 /// ("Mobile uses one column of grouped lists. Desktop uses a 2-column grid
@@ -31,11 +31,13 @@ class SettingsSection extends StatelessWidget {
       return NooGroupedList(
         label: title,
         footer: subtitle != null ? Text(subtitle!) : null,
-        // Every mobile Settings section is individually collapsible,
-        // expanded by default - replaces the old trailing jump rail (see
-        // `account_view.dart`'s doc comment) as the way to navigate a long
-        // Settings screen quickly.
-        collapsible: true,
+        // Not collapsible: each mobile Settings section now renders on its
+        // own pushed screen (see `account_view.dart`'s doc comment for the
+        // menu-then-pushed-screen pattern), so there's no long single-scroll
+        // page left to collapse sections *within* - an earlier design had
+        // every section inline in one column and made them individually
+        // collapsible for exactly that reason; that's gone now that each
+        // one is already isolated on its own screen.
         children: children,
       );
     }