feat: enhance mobile responsiveness with collapsible controls and automation/effects bars

Added comprehensive mobile support for Phase 15 (Polish & Optimization):

**Mobile Layout Enhancements:**
- Track controls now collapsible on mobile with two states:
  - Collapsed: minimal controls with expand chevron, R/M/S buttons, horizontal level meter
  - Expanded: full height fader, pan control, all buttons
- Track collapse buttons added to mobile view (left chevron for track collapse, right chevron for control collapse)
- Master controls collapse button hidden on desktop (lg:hidden)
- Automation and effects bars now available on mobile layout
- Both bars collapsible with eye/eye-off icons, horizontally scrollable when zoomed
- Mobile vertical stacking: controls → waveform → automation → effects per track

**Bug Fixes:**
- Fixed track controls and waveform container height matching on desktop
- Fixed Modal component prop: isOpen → open in all dialog components
- Fixed TypeScript null check for audioBuffer.duration
- Fixed keyboard shortcut category: 'help' → 'view'

**Technical Improvements:**
- Consistent height calculation using trackHeight variable
- Proper responsive breakpoints with Tailwind (sm:640px, lg:1024px)
- Progressive disclosure pattern for mobile controls

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/dialogs/KeyboardShortcutsDialog.tsx

@@ -0,0 +1,140 @@
+'use client';
+
+import * as React from 'react';
+import { Keyboard, X } from 'lucide-react';
+import { Modal } from '@/components/ui/Modal';
+import { Button } from '@/components/ui/Button';
+import { cn } from '@/lib/utils/cn';
+
+export interface KeyboardShortcutsDialogProps {
+  open: boolean;
+  onClose: () => void;
+}
+
+interface ShortcutCategory {
+  name: string;
+  shortcuts: Array<{
+    keys: string[];
+    description: string;
+  }>;
+}
+
+const SHORTCUTS: ShortcutCategory[] = [
+  {
+    name: 'Playback',
+    shortcuts: [
+      { keys: ['Space'], description: 'Play / Pause' },
+      { keys: ['Home'], description: 'Go to Start' },
+      { keys: ['End'], description: 'Go to End' },
+      { keys: ['←'], description: 'Seek Backward' },
+      { keys: ['→'], description: 'Seek Forward' },
+      { keys: ['Ctrl', '←'], description: 'Seek Backward 5s' },
+      { keys: ['Ctrl', '→'], description: 'Seek Forward 5s' },
+    ],
+  },
+  {
+    name: 'Edit',
+    shortcuts: [
+      { keys: ['Ctrl', 'Z'], description: 'Undo' },
+      { keys: ['Ctrl', 'Shift', 'Z'], description: 'Redo' },
+      { keys: ['Ctrl', 'X'], description: 'Cut' },
+      { keys: ['Ctrl', 'C'], description: 'Copy' },
+      { keys: ['Ctrl', 'V'], description: 'Paste' },
+      { keys: ['Delete'], description: 'Delete Selection' },
+      { keys: ['Ctrl', 'D'], description: 'Duplicate' },
+      { keys: ['Ctrl', 'A'], description: 'Select All' },
+    ],
+  },
+  {
+    name: 'View',
+    shortcuts: [
+      { keys: ['Ctrl', '+'], description: 'Zoom In' },
+      { keys: ['Ctrl', '-'], description: 'Zoom Out' },
+      { keys: ['Ctrl', '0'], description: 'Fit to View' },
+    ],
+  },
+  {
+    name: 'File',
+    shortcuts: [
+      { keys: ['Ctrl', 'S'], description: 'Save Project' },
+      { keys: ['Ctrl', 'K'], description: 'Open Command Palette' },
+    ],
+  },
+];
+
+function KeyboardKey({ keyName }: { keyName: string }) {
+  return (
+    <kbd className="px-2 py-1 text-xs font-semibold bg-muted border border-border rounded shadow-sm min-w-[2rem] text-center inline-block">
+      {keyName}
+    </kbd>
+  );
+}
+
+export function KeyboardShortcutsDialog({ open, onClose }: KeyboardShortcutsDialogProps) {
+  if (!open) return null;
+
+  return (
+    <Modal open={open} onClose={onClose} title="">
+      <div className="p-6 max-w-2xl">
+        {/* Header */}
+        <div className="flex items-start justify-between mb-6">
+          <div className="flex items-center gap-3">
+            <Keyboard className="h-6 w-6 text-primary" />
+            <h2 className="text-xl font-semibold">Keyboard Shortcuts</h2>
+          </div>
+          <button onClick={onClose} className="text-muted-foreground hover:text-foreground">
+            <X className="h-5 w-5" />
+          </button>
+        </div>
+
+        {/* Shortcuts Grid */}
+        <div className="grid grid-cols-1 md:grid-cols-2 gap-6">
+          {SHORTCUTS.map((category) => (
+            <div key={category.name} className="space-y-3">
+              <h3 className="text-sm font-semibold text-primary border-b border-border pb-2">
+                {category.name}
+              </h3>
+              <div className="space-y-2">
+                {category.shortcuts.map((shortcut, index) => (
+                  <div
+                    key={index}
+                    className="flex items-center justify-between gap-4 py-1.5"
+                  >
+                    <span className="text-sm text-foreground flex-1">
+                      {shortcut.description}
+                    </span>
+                    <div className="flex items-center gap-1 flex-shrink-0">
+                      {shortcut.keys.map((key, keyIndex) => (
+                        <React.Fragment key={keyIndex}>
+                          {keyIndex > 0 && (
+                            <span className="text-muted-foreground text-xs mx-0.5">+</span>
+                          )}
+                          <KeyboardKey keyName={key} />
+                        </React.Fragment>
+                      ))}
+                    </div>
+                  </div>
+                ))}
+              </div>
+            </div>
+          ))}
+        </div>
+
+        {/* Footer */}
+        <div className="mt-6 pt-4 border-t border-border">
+          <p className="text-xs text-muted-foreground text-center">
+            Press <KeyboardKey keyName="Ctrl" /> + <KeyboardKey keyName="K" /> to open the
+            command palette and search for more actions
+          </p>
+        </div>
+
+        {/* Close Button */}
+        <div className="mt-6 flex justify-end">
+          <Button onClick={onClose} variant="default">
+            Close
+          </Button>
+        </div>
+      </div>
+    </Modal>
+  );
+}