Fix all clippy warnings across workspace

collapsible_if, let-chains, derivable_impls, clone_on_copy, useless_vec, approx_constant, io_other_error, single_match, collapsible_match, unnecessary_map_or, manual_is_multiple_of, result_large_err, too_many_arguments, upper_case_acronyms

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/xserial-gui/src/ui_fonts.rs

@@ -10,19 +10,14 @@ use tracing::{info, warn};
 
 const FONT_SETTINGS_FILE_NAME: &str = "gui-fonts.json";
 
-#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize, Default)]
 pub enum FontChoice {
     Auto,
+    #[default]
     Default,
     System(String),
 }
 
-impl Default for FontChoice {
-    fn default() -> Self {
-        Self::Default
-    }
-}
-
 #[derive(Clone, Debug)]
 pub struct FontCandidate {
     pub id: String,