feat(ui): implement v2 three-panel layout

- Collapsible sidebar (68px icons / 230px expanded, toggle via logo)
- LibraryPanel: sessions expand inline to show tracks, search + filter chips
- PlayerPanel: extracted from SongPage, used as embeddable panel
- BandPage: Library + Player side by side; song selection via ?song= URL param
- SongPage: thin wrapper around PlayerPanel (kept for direct deep-links)
- CSS palette updated to v2 violet/cyan/emerald scheme
- Mobile (<900px): BandPage shows library or player, never both

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

web/src/index.css

@@ -16,23 +16,30 @@ input, textarea, button, select {
 
 /* ── Design system (dark only — no light mode in v1) ─────────────────────── */
 :root {
-  --bg:          #0f0f12;
+  /* v2 dark-space palette */
+  --bg:          #0c0e1a;
+  --bg-card:     #10131f;
+  --bg-raised:   #151828;
+  --bg-hover:    #1a1e30;
   --bg-subtle:   rgba(255,255,255,0.025);
   --bg-inset:    rgba(255,255,255,0.04);
-  --border:      rgba(255,255,255,0.08);
-  --border-subtle: rgba(255,255,255,0.05);
-  --text:        #eeeef2;
-  --text-muted:  rgba(255,255,255,0.35);
-  --text-subtle: rgba(255,255,255,0.22);
-  --accent:      #e8a22a;
-  --accent-hover: #f0b740;
-  --accent-bg:   rgba(232,162,42,0.1);
-  --accent-border: rgba(232,162,42,0.28);
-  --accent-fg:   #0f0f12;
-  --teal:        #4dba85;
-  --teal-bg:     rgba(61,200,120,0.1);
-  --danger:      #e07070;
-  --danger-bg:   rgba(220,80,80,0.1);
+  --border:      rgba(255,255,255,0.06);
+  --border-bright: rgba(255,255,255,0.12);
+  --border-subtle: rgba(255,255,255,0.04);
+  --text:        #e8e9f0;
+  --text-muted:  rgba(232,233,240,0.55);
+  --text-subtle: rgba(232,233,240,0.28);
+  /* Violet accent */
+  --accent:      #8b5cf6;
+  --accent-light: #a78bfa;
+  --accent-hover: #9f70f8;
+  --accent-bg:   rgba(139,92,246,0.12);
+  --accent-border: rgba(139,92,246,0.3);
+  --accent-fg:   #ffffff;
+  --teal:        #34d399;
+  --teal-bg:     rgba(52,211,153,0.1);
+  --danger:      #f43f5e;
+  --danger-bg:   rgba(244,63,94,0.1);
 }
 
 /* ── Responsive Layout ──────────────────────────────────────────────────── */