Fix race condition in timeline drag handlers when finding added media items

Description

A race condition exists in the timeline drag handlers when processing dropped files. The current implementation searches for added media items by matching name and URL, which can be unreliable if multiple items have the same name or if the state hasn't updated yet.

Location

File: apps/web/src/components/editor/timeline/timeline-drag-handlers.tsx
Lines: ~115-124

Issue

The code currently finds added media items like this:

const addedItem = currentMediaItems.find(
    (item) =>
        item.name === processedItem.name &&
        item.url === processedItem.url,
);

This approach can fail when:

• Multiple items have the same name
• State updates are delayed
• Race conditions occur during processing

Proposed Solution

Use the returned ID from addMediaItem instead of searching by name and URL:

const mediaItemId = await addMediaItem(activeProject.id, processedItem);
const addedItem = currentMediaItems.find(
    (item) => item.id === mediaItemId
);

References

• PR: https://github.com/OpenCut-app/OpenCut/pull/324
• Comment: https://github.com/OpenCut-app/OpenCut/pull/324#discussion_r2211935392

Reported by: @simonorzel26

Start in apps/web/src/components/editor/timeline/timeline-drag-handlers.tsx around lines 115-124 and inspect how addMediaItem is called and how the returned value is used. Review PR #324 and its discussion for context. Done means dropped media items are identified reliably even with duplicate names or delayed state updates.