(studio) Add I/O markers and "A/E" navigation shortcuts to the Studio timeline #807

Problem

Navigating long compositions is currently manual and inefficient. There is also no way to isolate a specific segment of the timeline for focused editing or looping. This makes it difficult to fine-tune specific transitions or "beats" without playing the entire video from the start.

Use Case:

• Quickly jumping between the start and end of a project for review.
• Isolating a 2-second transition (e.g., frames 30-90) to check animation fluidness without manual scrubbing.
• Defining a specific "Work Area" for focused looping during the design phase.

Proposed solution

1. Navigation Shortcuts

Implement global event listeners for the following keys to improve timeline traversal:

• A: Jump playhead to the Start Point (Global start or custom In-point).
• E: Jump playhead to the End Point (Global end or custom Out-point).

2. Adjustable Work Area (I/O Markers)

Introduce a "Work Area" logic that constrains the playback engine:

• Setting Points: Use I to set the In-point and O to set the Out-point at the current playhead position.
• Constrained Navigation: When these points are set, the A and E shortcuts should jump to these markers instead of the absolute project boundaries.
• Looping Logic: When the loop toggle is active, the playhead should immediately seek back to the In-point upon reaching the Out-point.