feat: Add integrated mixer panel in session view

[ChuxiJ]

Summary

Ableton's session view has an integrated mixer section below the clip grid showing volume, pan, sends, solo, mute, and arm per track. Our session view has no mixer — users must switch to the arrangement/mixer view to adjust levels.

Proposed Design

Mixer Strip (per track, below clip grid)

• Volume fader: Vertical mini-fader or horizontal slider
• Pan knob: Stereo panning control
• Solo button (S): Solo this track
• Mute button (M): Mute this track
• Arm button (R): Record-enable this track (red when armed)
• Level meter: Real-time output level visualization
• Send knobs: One per return/bus track (if applicable)

Layout

• Mixer appears below the clip grid, aligned with track columns
• Collapsible/toggleable with a keyboard shortcut
• Scrolls horizontally with the clip grid

Sync with Existing Mixer

• Uses the same store state as the arrangement view mixer
• Changes in either view reflect immediately in the other
• No duplicate state — single source of truth

Acceptance Criteria

• Volume fader per track below clip grid
• Pan control per track
• Solo/Mute/Arm buttons per track
• Level meter showing real-time output
• Mixer panel toggleable with keyboard shortcut
• Horizontally synced scroll with clip grid
• State synced with arrangement view mixer (same store)
• Master track volume/meter at the right edge
• Responsive: mixer collapses gracefully on small screens
• E2E test: mixer changes during session playback