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

[beezly]

Summary

• MessageImageView's .task block checked NSCache on cache hit but never returned, causing getMediaContent to be called on every channel visit even for already-cached images
• Adds the missing return after a cache hit — a one-line fix matching the pattern already used in MatrixImageView and AvatarImage
• Verified with OSLog debug instrumentation: before the fix, every return to a channel showed cache HIT immediately followed by a redundant network fetch; after the fix, cache hits return early with no network call

Test plan

• Navigate to a channel with images and note memory usage
• Switch to another channel and back — memory should no longer increase for already-loaded images
• First load of an image still fetches from network (cache miss path unchanged)

Fixes #89

[beezly]

Superseded by a clean branch based on origin/main — see new PR.