Make it easier to fit sequence attachments to a desired duration

Currently, sequence attachments are configured by specifying a starting frame and a playback FPS. The ending frame is determined automatically from those values.

For some workflows, especially those using slider constraints or other parameter-driven animation, this can make it cumbersome to fit a sequence to a desired duration. If the duration of the driving animation changes, the sequence FPS often has to be recalculated manually.

Possible approaches could include (but are not limited to):

- Automatically deriving the playback FPS from a specified duration.
- Providing a command or tool to fit a sequence to the current key duration.
- Any other workflow that reduces manual FPS calculations while remaining consistent with the rest of the editor.

The goal is not necessarily to replace the current FPS-based workflow, but to provide an easier way to synchronize sequence playback with the duration of an animation when needed.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30385-%E5%85%B3%E4%BA%8E%E6%BB%91%E5%9D%97%E7%BA%A6%E6%9D%9F%E6%98%A0%E5%B0%84%E5%85%B3%E7%B3%BB%E7%9A%84%E4%B8%80%E4%BA%9B%E6%83%B3%E6%B3%95/

Start by reviewing how sequence attachments currently use a starting frame and playback FPS, then read the related forum thread for workflow context. Define a single duration-based synchronization approach that remains compatible with the existing FPS workflow; done means the desired animation duration can be used without manual FPS recalculation.

Written by the indexing model from the issue text.