chore(backend): satisfy cargo fmt and clippy --all-targets

- Apply rustfmt diffs in claude_desktop_config.rs
- Allow needless_return on current_platform_paths (cfg-mirrored arms)
- Allow too_many_arguments on RequestForwarder::forward
- Replace `let mut + reassign` with struct literals in tests
  (settings, backup, provider, response_processor)
- Use Path::new instead of PathBuf::from to fix cmp_owned in misc tests
- Replace 3.14 with 3.5 in config test to avoid approx_constant lint

src-tauri/src/database/backup.rs

@@ -791,8 +791,10 @@ mod tests {
         std::fs::create_dir_all(&test_home).expect("create test home");
         std::env::set_var("CC_SWITCH_TEST_HOME", &test_home);
 
-        let mut settings = AppSettings::default();
-        settings.backup_interval_hours = Some(0);
+        let settings = AppSettings {
+            backup_interval_hours: Some(0),
+            ..AppSettings::default()
+        };
         update_settings(settings).expect("disable auto backup");
 
         let db = Database::memory()?;