fix: snapshot lastModified before sort in TelegramCacheManager

[faraz152]

Problem

TelegramCacheManager.trimEmbeddedArtCache() sorted embedded art files with:

embeddedArtFiles.sortBy { it.lastModified() }

File.lastModified() is called for every pairwise comparison during the sort. If a concurrent coroutine touches any of those files (e.g. a Telegram thumbnail download calling setLastModified()), the same file can return a different timestamp in a later comparison — breaking TimSort's transitivity requirement and crashing with:

java.lang.IllegalArgumentException: Comparison method violates its general contract!
    at java.util.TimSort.mergeLo

This is the same root cause as the crash reported in #1886 ("app crash when scanning .lrc"), which occurs during the broad sync sweep that runs album-art caching and Telegram operations in parallel.

Fix

Snapshot all lastModified() timestamps into an immutable map before entering the sort, so the comparator always sees a stable value:

val snapshots = embeddedArtFiles.associateWith { it.lastModified() }
embeddedArtFiles.sortWith(compareBy { snapshots[it] })

This is exactly the pattern already applied to AlbumArtCacheManager (snapshotFilesForCleanup).

Test plan

• Trigger a full library sync while Telegram channel is active — no IllegalArgumentException in logs
• Verify LRU eviction still deletes oldest embedded art files when cache exceeds limit

Closes #1886