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>

.claude/context/styling.md

@@ -90,8 +90,12 @@ Gotchas:
 - `NooGroupedList` draws dividers by showing `line` through 1px gaps, so each
   child must paint its own surface (`NooSettingsRow` and `NooTabOrderRow`
   do). Its `collapsible`/`initiallyExpanded` params (off by default) make
-  `label` a tap target that shows/hides the card - `SettingsSection` is the
-  only caller that opts in, for Settings' mobile sections.
+  `label` a tap target that shows/hides the card - no current caller opts
+  in (Settings' mobile sections used to, when every section rendered
+  inline in one long column; now each section is its own pushed screen -
+  see `account_view.dart`'s doc comment - so there's nothing left to
+  collapse). The params stay on the component itself since it's otherwise
+  generic.
 - `NooSwipeAction` only reveals its action. The user has to tap the block to
   trigger it; a full swipe never deletes.
 - Window chrome (macOS traffic lights, the Windows 40px title bar) isn't