fix(android): smoother Erkunden swipe + top charts no longer vertically centered

- Pre-compose the neighbour page (beyondViewportPageCount = 1) so the first
  swipe doesn't hitch while the heavy LazyColumn builds.
- Drive the TabRow indicator off pagerState.currentPage so it moves with the
  swipe (at the half-way point) instead of only on settle — the lag read as
  "not smooth".
- Wrap each pager page in Box(fillMaxSize) so short content (the Top-Alben
  charts row) sits at the top again, not vertically centered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/screen/ExploreScreen.kt

@@ -163,20 +163,22 @@ fun ExploreScreen(
 
     Column(Modifier.fillMaxSize()) {
         // ── Tabs ─────────────────────────────────────────────────────────────
+        // Drive the indicator off the pager's current page so it moves WITH the swipe (at the
+        // half-way point), not only once the swipe settles — that lag read as "not smooth".
         TabRow(
-            selectedTabIndex = state.tab.ordinal,
+            selectedTabIndex = pagerState.currentPage,
             containerColor   = MaterialTheme.colorScheme.background,
             contentColor     = NsAccent,
         ) {
             Tab(
-                selected = state.tab == ExploreTab.LIBRARY,
+                selected = pagerState.currentPage == ExploreTab.LIBRARY.ordinal,
                 onClick  = { vm.selectTab(ExploreTab.LIBRARY); vm.stopPreview() },
                 text     = { Text("Bibliothek", maxLines = 1) },
                 selectedContentColor   = NsAccent,
                 unselectedContentColor = MaterialTheme.colorScheme.onSurfaceVariant,
             )
             Tab(
-                selected = state.tab == ExploreTab.GLOBAL,
+                selected = pagerState.currentPage == ExploreTab.GLOBAL.ordinal,
                 onClick  = { vm.selectTab(ExploreTab.GLOBAL); vm.stopPreview() },
                 text     = { Text("Erkunden", maxLines = 1) },
                 selectedContentColor   = NsAccent,
@@ -246,8 +248,14 @@ fun ExploreScreen(
         // ── Content (swipeable) ────────────────────────────────────────────────
         HorizontalPager(
             state = pagerState,
+            // Pre-compose the neighbour page so the swipe doesn't hitch while its (heavy) LazyColumn
+            // builds on first drag.
+            beyondViewportPageCount = 1,
             modifier = Modifier.weight(1f),
         ) { page ->
+            // fillMaxSize so short content (e.g. the Top-Alben charts row) sits at the top, not
+            // vertically centered in the page.
+            Box(Modifier.fillMaxSize()) {
                 when (page) {
                     ExploreTab.LIBRARY.ordinal -> LibrarySearchContent(
                         query       = state.query,
@@ -270,6 +278,7 @@ fun ExploreScreen(
                 }
             }
         }
+    }
 }
 
 // ── Library (local Navidrome) search ─────────────────────────────────────────