Remove scrollbar thumb and restructure Settings profile card

- Revert the scrollbar-thumb feature entirely (per-view Scrollbar
  wrappers and the ScrollbarThemeData override) - back to plain
  scrollables with no visible thumb
- Settings: move the logout button into the profile card (next to the
  name/email), and move the ONLINE/OFFLINE badge + "Connected to..."
  text out of the top row into a combined line at the bottom of the
  card, instead of split between the AppBar and the top of the screen
- Update architecture.md/styling.md to drop the stale scrollbar-theme
  documentation

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

.claude/context/architecture.md

@@ -101,8 +101,8 @@ owns the app's share-intent listener (`receive_sharing_intent`): both
 Five of the six tabs (all but Files) plus each tab's own controls share
 [`SyncedHeaderScaffold`](../../lib/widgets/synced_header_scaffold.dart) — a
 `CustomScrollView` with a pull-down "sync status" header (Google
-Photos-style), a `Scrollbar`, and a classic Material refresh spinner shown
-during a pull-triggered sync. `SearchView`, `AccountView` (Settings), the
+Photos-style) and a classic Material refresh spinner shown during a
+pull-triggered sync. `SearchView`, `AccountView` (Settings), the
 file-details sheet, the share sheet, and `ShareUploadView` (the
 share-to-upload destination picker) are pushed on top via
 `Navigator`/`showModalBottomSheet`/`showGradualBottomSheet` rather than