refactor(layout): replace two-pane split with single-pane navigation

Library and PlayerPanel now display one at a time on all screen sizes.
Selecting a song navigates to the player; the back button returns to
the library. Removes isMobile breakpoint logic and fixed 340px panel width.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/components/LibraryPanel.tsx

@@ -277,13 +277,10 @@ export function LibraryPanel({ bandId, selectedSongId, onSelectSong }: LibraryPa
 
   return (
     <div style={{
-      width: 340,
+      flex: 1,
-      minWidth: 280,
-      flexShrink: 0,
       display: "flex",
       flexDirection: "column",
       background: "#10131f",
-      borderRight: `1px solid ${border}`,
       height: "100%",
       overflow: "hidden",
     }}>

web/src/pages/BandPage.tsx

@@ -1,28 +1,9 @@
-import { useState, useEffect } from "react";
 import { useParams, useSearchParams } from "react-router-dom";
 import { useQuery } from "@tanstack/react-query";
 import { getBand } from "../api/bands";
 import { LibraryPanel } from "../components/LibraryPanel";
 import { PlayerPanel } from "../components/PlayerPanel";
 
-// ── Empty player state ────────────────────────────────────────────────────────
-
-function EmptyPlayer() {
-  return (
-    <div style={{ flex: 1, display: "flex", flexDirection: "column", alignItems: "center", justifyContent: "center", background: "#0c0e1a", gap: 12, padding: 40 }}>
-      <div style={{ width: 64, height: 64, borderRadius: 20, background: "rgba(139,92,246,0.08)", border: "1px solid rgba(139,92,246,0.15)", display: "flex", alignItems: "center", justifyContent: "center" }}>
-        <svg width="28" height="28" viewBox="0 0 28 28" fill="none">
-          <path d="M7 14c0-4.5 3.5-7 7-7s7 2.5 7 7v5L23 22H5l2-3v-5z" stroke="rgba(139,92,246,0.5)" strokeWidth="1.5" strokeLinejoin="round" />
-          <path d="M10 22a4 4 0 0 0 8 0" stroke="rgba(139,92,246,0.5)" strokeWidth="1.5" />
-        </svg>
-      </div>
-      <p style={{ fontSize: 13, color: "rgba(232,233,240,0.28)", margin: 0, textAlign: "center" }}>
-        Select a track from the library to start listening
-      </p>
-    </div>
-  );
-}
-
 // ── BandPage ──────────────────────────────────────────────────────────────────
 
 export function BandPage() {
@@ -31,15 +12,6 @@ export function BandPage() {
   // selectedSongId is kept in URL as ?song=<id> so deep-links and browser back work
   const selectedSongId = searchParams.get("song");
 
-  const [isMobile, setIsMobile] = useState(false);
-
-  useEffect(() => {
-    const check = () => setIsMobile(window.innerWidth < 900);
-    check();
-    window.addEventListener("resize", check);
-    return () => window.removeEventListener("resize", check);
-  }, []);
-
   const { data: band, isLoading } = useQuery({
     queryKey: ["band", bandId],
     queryFn: () => getBand(bandId!),
@@ -57,9 +29,6 @@ export function BandPage() {
   if (isLoading) return <div style={{ color: "rgba(232,233,240,0.35)", padding: 32 }}>Loading…</div>;
   if (!band || !bandId) return <div style={{ color: "#f87171", padding: 32 }}>Band not found</div>;
 
-  // ── Mobile: show library OR player, not both ──────────────────────────────
-
-  if (isMobile) {
   if (selectedSongId) {
     return (
       <div style={{ height: "100%", display: "flex", flexDirection: "column" }}>
@@ -72,6 +41,7 @@ export function BandPage() {
       </div>
     );
   }
+
   return (
     <div style={{ height: "100%", display: "flex", flexDirection: "column" }}>
       <LibraryPanel
@@ -82,27 +52,3 @@ export function BandPage() {
     </div>
   );
 }
-
-  // ── Desktop: three-panel (Sidebar is handled by AppShell, we add Library + Player) ──
-
-  return (
-    <div style={{ display: "flex", height: "100%", overflow: "hidden" }}>
-      <LibraryPanel
-        bandId={bandId}
-        selectedSongId={selectedSongId}
-        onSelectSong={selectSong}
-      />
-
-      {selectedSongId ? (
-        <PlayerPanel
-          key={selectedSongId}
-          songId={selectedSongId}
-          bandId={bandId}
-          onBack={clearSong}
-        />
-      ) : (
-        <EmptyPlayer />
-      )}
-    </div>
-  );
-}