Add --clipboard flag to read images from system clipboard

Add a `--clipboard` / `-c` flag to ocrs-cli that reads images from the
system clipboard instead of a file path. This feature requires the
`clipboard` Cargo feature to be enabled at build time (uses the arboard
crate).

When built without the feature, the flag is still accepted but returns a
helpful error message directing users to rebuild with the feature enabled.

Supersedes #128.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: robertknight