fix(theme): match Herdr default palette

[RaviTharuma]

Summary

• use Catppuccin when Herdr theme inheritance is enabled and no theme name is configured
• keep One Light as the explicit opt-out palette when inheritance is disabled
• cover the unset-name behavior with a regression test

Fixes #5

Verification

• cargo test theme::tests (4 passed)
• cargo check
• full cargo test (66 passed; 2 pre-existing app tests fail because they invoke the unavailable herdr executable)
• formatting check unavailable because the active Rust toolchain does not have rustfmt installed

🤖 Generated with Claude Code

[RaviTharuma]

Closing as a duplicate of #9, which has the same fix with broader regression coverage for missing/unknown theme names. Please review #9 instead: #9