Standardize playback shortcuts

[austin-smith]

What changed

• Keep bare left and right arrows focused on seeking by five seconds.
• Add Ctrl+Left and Ctrl+Right as previous/next track aliases while retaining p and n.
• Ignore held-key repeats for discrete track changes while allowing repeated seeking.
• Document p/n as the compact canonical previous/next binding in the keymap overlay.
• Add unit and headless terminal-input coverage for modifier handling and decoded arrow sequences.

Why

Bare arrows conventionally adjust position within the current media item, while a modifier distinguishes track navigation. Centralizing transport matching also prevents unsupported modifier combinations from being consumed accidentally.

User impact

Users can continue using p/n, use bare arrows to seek, or use Ctrl+arrows to change tracks. The persistent playback footer is unchanged.

Validation

• bun run typecheck
• bun test — 1,047 passed, 0 failed