Fix memory leak: return early on image cache hit in MessageImageView

[beezly]

Summary

Fixes the memory leak where getMediaContent was called on every channel visit even for already-cached images.

Two-layer caching design

NSCache (checked in init): synchronously pre-populates image before the view appears, preventing flicker on scroll/revisit. Also skips the expensive decode step (toOrientedImage) on repeat loads.

SDK media cache (getMediaContent): avoids network re-fetches. Now always called in .task so imageData is populated for drag-and-drop, regardless of whether the decoded NSImage is already cached.

The image is already on screen (via init) before .task runs its disk read, so flicker prevention is unaffected.

Re: drag-and-drop regression

The original version of this PR returned early on a cache hit without populating imageData, breaking drag-and-drop. This revision always calls getMediaContent to ensure imageData is set, but still skips the decode step when the NSImage is already in the cache.

Test plan

• Navigate to a channel with images — images load without flicker
• Switch channels and back — no memory growth, images still appear immediately
• Drag an image to the desktop on first load (cache miss) — file is copied correctly
• Drag an image to the desktop after switching channels and back (cache hit) — file is copied correctly

Fixes #89

[viktorstrate]

This is a nice fix generally, but it breaks drag and drop to download images. If you try to drag an image to the desktop it will not copy the file, because the imageData variable is never set.

For preview, it still works, since the previewImage function will download the image if imageData is not set.
I think we can do something similar for drag and drop maybe?