Add "Search in bottom bar" to Settings

New Settings > Appearance toggle moves Search into the bottom bar and
out of the top bar, so there's only one search entry point on screen.
Tapping it pushes SearchView same as before - it's never highlighted,
since it isn't a real destination.

- Attached: Search becomes the row's last item, styled like any other
  tab but permanently idle.
- Floating: Search gets its own separate, always fully round satellite
  circle beside the bar instead.

Enabling it costs one regular tab: SettingsController.maxVisibleTabs
is now a dynamic getter (defaultMaxVisibleTabs - 1 while the toggle is
on) instead of the old flat constant, re-enforced immediately if the
user already has all 5 slots filled - the same cleanup path used for
a fresh install or an app update that added a new tab.

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

lib/models/app_tab.dart

@@ -16,8 +16,13 @@ enum AppTab {
 }
 
 /// At most this many tabs may be visible in the bottom nav bar at once —
-/// the rest are reachable through the "more" dropdown instead.
-const int maxVisibleTabs = 5;
+/// the rest are reachable through the "more" dropdown instead. The
+/// effective cap a screen should actually use is
+/// [SettingsController.maxVisibleTabs], not this directly - it's one
+/// lower than this while `searchInBottomBar` is on, since Search then
+/// takes the freed-up slot itself rather than competing with the user's
+/// own reorderable tabs for one of these.
+const int defaultMaxVisibleTabs = 5;
 
 extension AppTabInfo on AppTab {
   String get label {