yoda: warning cleanup sweep (dead code + clippy --fix) (squashed)

Squash of 4 yoda commits:
- 84437e21 yoda: libcosmic-yoda dead-code purge (14->0 warnings)
- 999db0a4 yoda: cosmic-theme cleanup (4->0 warnings) — workspace at 0 warnings total
- 4743bb8e yoda: cargo clippy --fix on libcosmic-yoda (115->33 warnings)
- 675f3b59 chore: reduce local stack warnings

cosmic-theme/Cargo.toml

@@ -35,6 +35,3 @@ thiserror.workspace = true
 [dev-dependencies]
 insta = "1.47.2"
 
-[profile.dev.package]
-insta.opt-level = 3
-similar.opt-level = 3