refactor(styles): extract .app-scrollbar utility, apply to channel lists

Move the always-visible themed scrollbar from epg-list into a global
.app-scrollbar utility class in styles.scss. Apply it to the All
channels and Groups view virtual-scroll viewports so the channel list
gets the same persistent gutter and themed thumb as the EPG list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: c470abf78379

apps/web/src/styles.scss

@@ -27,6 +27,40 @@
 
 // ─── Shared Components ────────────────────────────────────────────────────────
 
+// Always-visible themed scrollbar. Apply to scroll containers that should
+// reserve gutter and not rely on macOS overlay scrollbars (which auto-fade).
+.app-scrollbar {
+    scrollbar-gutter: stable;
+    scrollbar-width: thin;
+    scrollbar-color: color-mix(in srgb, var(--app-muted-color) 55%, transparent)
+        transparent;
+
+    &::-webkit-scrollbar {
+        width: 8px;
+        height: 8px;
+    }
+
+    &::-webkit-scrollbar-track {
+        background: transparent;
+    }
+
+    &::-webkit-scrollbar-thumb {
+        background-color: color-mix(
+            in srgb,
+            var(--app-muted-color) 55%,
+            transparent
+        );
+        border-radius: 999px;
+        border: 2px solid transparent;
+        background-clip: content-box;
+
+        &:hover {
+            background-color: var(--app-muted-color);
+            background-clip: content-box;
+        }
+    }
+}
+
 app-root.macos-platform .current-playlist {
     padding-top: 30px;
 }