refactor(settings): split the dashboard layout section into draft, lists and hero settings

DashboardLayoutSection was a 620-line component. Its parts move to
components/settings/dashboard-layout/:

- layout-model: the reorder and hero-kind helpers and the label maps
- use-dashboard-layout-draft: the tokened read, the draft, save and
  reset (with the snapshot and digest refresh) and the draft edits
- hero-settings-fieldset: the top card's switch, lead and highlights
- widget-lists: the sortable web list, the native-only group and rows

The section composes them. Markup and behaviour are unchanged.

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
-  "count": 403,
-  "total": 139587,
+  "count": 402,
+  "total": 138967,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -231,7 +231,6 @@
     "components/settings/api-section.tsx :: ApiTokensCard": 366,
     "components/settings/coach-nudge-card.tsx :: CoachNudgeCard": 251,
     "components/settings/coach-prefs-section.tsx :: CoachPrefsSection": 350,
-    "components/settings/dashboard-layout-section.tsx :: DashboardLayoutSection": 620,
     "components/settings/environment-section.tsx :: EnvironmentSection": 666,
     "components/settings/import-panel/apple-health-import-card.tsx :: AppleHealthImportCard": 298,
     "components/settings/import-panel/csv-import-card.tsx :: CsvImportCard": 212,