feat: Multi-track rendering in NotationPanel

[ElBeh]

Description

Render multiple instrument tracks simultaneously in the NotationPanel
(e.g. guitar + bass). Useful during band practice when following several
parts at once. alphaTab supports this natively via api.renderTracks([...]).

Estimated effort: ~1 session (medium)

Requirements

• Track selector supports multi selection (checkboxes), at least one track required
• Persist selected track set per song (replaces single active track index)
• Tuning display: show tuning of the first selected track
• Verify sync cursor behavior with multiple rendered tracks
• Verify mixer interaction (mute/solo/volume) with multi-track rendering
• Band Sync: viewers select their track set locally, not synced (verify)
• Verify rendering performance with 3+ tracks in page layout
• Works in both layout modes, plays well with resizable panel height (#[Resize-Issue/PR])

Implementation

• src/components/Tabs/NotationPanel.tsx: track selector UI, renderTracks call
• Persistence: track set per song via config store or song metadata

Out of Scope

• Per-track layout options (staff visibility, transposition)
• Syncing track selection across Band Sync clients

Decisions

• Native renderTracks over multiple alphaTab instances: single API, shared cursor and player
• First selected track drives track-dependent features (tuning display) to keep behavior deterministic