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

Cargo.toml

@@ -269,5 +269,13 @@ tracing = "0.1"
 tokio = "1.52"
 zbus = {version = "5.15", default-features = false}
 
+# Speed up snapshot diffing in cosmic-theme tests. Cargo silently ignores
+# [profile.*] blocks in non-root manifests, so this lives at the
+# workspace root.
+[profile.dev.package.insta]
+opt-level = 3
+[profile.dev.package.similar]
+opt-level = 3
+
 [dev-dependencies]
 tempfile = "3.27.0"