fix(titlebar): hide stripped strip in plasma when empty

If the user has search-in-titlebar off AND no titlebar-nav items
enabled, plasma mode would render an empty 42px strip below the KWin
SSD. Gate shouldShowTitleBar() in plasma on whether either is enabled.
+page.svelte's search-location and titlebar-nav subscriptions now
also recompute showTitleBar so the layout reflows immediately when
the user toggles those settings.

cc

src/routes/+page.svelte

@@ -4848,6 +4848,10 @@
     initSearchBarLocation();
     const unsubscribeSearchBarLocation = subscribeSearchBarLocation(() => {
       searchBarLocationPref = getEffectiveSearchBarLocation();
+      // In plasma mode the strip's visibility depends on whether search
+      // or any nav item is enabled, so a search-location toggle can flip
+      // the strip on/off.
+      showTitleBar = shouldShowTitleBar();
     });
 
     // Initialize and subscribe to window controls customization
@@ -4868,6 +4872,10 @@
       tbNavLibrary = isLibraryInTitlebar();
       tbNavMyQbz = isMyQbzInTitlebar();
       tbNavPurchases = isPurchasesInTitlebar();
+      // In plasma mode the strip's visibility depends on whether any nav
+      // item or search is enabled, so toggling the last nav item can
+      // flip the strip on/off.
+      showTitleBar = shouldShowTitleBar();
     });
 
     // Detect floating window state (not maximized) for rounded corners + shadow