memory leak: screenshot cache never cleared after popup closes

[codehakase]

Summary

The SCREENSHOT_CACHE HashMap in src-tauri/src/lib.rs stores every captured screenshot in memory indefinitely. Images are only removed when the user explicitly clicks the Delete button. All other workflows—auto-dismiss, save, copy, edit—leave the image data in the cache.
The cache has three insertion points (capture_screenshot_internal, capture_screenshot_internal_with_auto_copy, capture_full_screen) but only one removal path (delete_from_memory command), which is only triggered by the delete button handler in useScreenshot.ts.

Impact

During long-running sessions, memory usage grows with every screenshot. Based on recent logs, individual images range from 38KB to 1.3MB. There is no automatic eviction—the cache only clears when the process exits.