Redesign the Image avatar tab from a single-image chooser into a grid over a
persistent image library (mirrors the Icon tab):
- Images the user adds live in <config>/contact-avatars/ and PERSIST as a
reusable, portable set — they travel with the wallet data dir, so avatars
survive moving to another machine without remembering source paths.
- The grid's first cell is always the "+ add image" button (opens the picker,
decode-verifies, copies into the library, auto-selects the new image).
- Each library image is a selectable thumbnail (Primary ring when selected)
with a delete badge (top-right, red on hover) to remove it from the library;
the delete is deferred past the grid loop and clears the selection if it
pointed at the removed file. Contacts still referencing a deleted image fall
back to their Z/T badge.
- Removes the previous auto-prune of "unused" images on edit/delete — images
are only removed by the explicit delete badge now.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>