Section loop for practice mode

[LoggeL]

Problem

Learning a song means repeating tricky sections over and over. Currently there's no way to loop a specific part — users have to manually seek back each time.

Proposal

An A-B loop feature: set a start point (A) and end point (B), and the player automatically loops that section.

Interaction

1. During playback, click Set A (or press [) → marks loop start at current time
2. Click Set B (or press ]) → marks loop end, loop immediately activates
3. Playback loops between A and B indefinitely
4. Click Clear Loop (or press \) to exit loop mode
5. Visual: highlighted region on the progress bar between A and B markers

Visual Indicators

• Two draggable markers on the progress bar (A and B)
• Shaded/highlighted region between them
• Markers can be fine-tuned by dragging after placement
• Loop count indicator: "Loop 3/∞"

Smart Defaults

• Clicking a lyrics segment while in "loop mode" could auto-set A/B to that segment's start/end
• Double-click a segment to loop just that line

Keyboard Shortcuts

• [ — Set A at current time
• ] — Set B at current time
• \ — Clear loop
• Hold Shift + click a word → Set A; Shift + click another word → Set B

Implementation

• In the seek/playback logic (usePlayer.ts), check if currentTime >= loopB and seek to loopA
• Store loop state: { a: number | null, b: number | null, active: boolean }
• Progress bar component needs overlay for the loop region

Complexity

Low-medium. Core logic is a simple time check in the playback loop. UI work for the progress bar markers is the main effort.