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,
-      minWidth: 280,
-      flexShrink: 0,
+      flex: 1,
       display: "flex",
       flexDirection: "column",
       background: "#10131f",
-      borderRight: `1px solid ${border}`,
       height: "100%",
       overflow: "hidden",
     }}>