feat: download and publish selected sticker packs

[9Ashwin]

Problem

packs install only supported preflight and could not materialize a selected pack for offline use.

Change

• Implement bounded, cancellable image transfers for local and HTTPS sources with retry handling.
• Stage and verify each original before acquiring the library write lock.
• Re-read personal and installed state under an exclusive lock before publishing images and the pack state atomically.
• Preserve usable prior state on transfer or commit failures and reuse verified staged/original files on retry.
• Wire the real install path into the CLI while keeping dry-run read-only.
• Add coverage for selective requests, exact GIF bytes, cancellation, retry reuse, repeat installs, and concurrent publication.

Closes #6

Validation

• GOTOOLCHAIN=auto go test -race ./...
• GOTOOLCHAIN=auto go vet ./...
• GOTOOLCHAIN=auto make quality
• git diff --check