Fix clippy unnecessary_sort_by lints from Rust 1.95

CI runs clippy with -D warnings. Rust 1.95 promoted
clippy::unnecessary_sort_by from a warn to a hard error in two
pre-existing call sites (sort_by + .cmp(&a) reversed for descending).
Replace with sort_by_key + std::cmp::Reverse, which is what clippy
suggests and is exactly equivalent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ndbroadbent

src-tauri/src/export.rs

@@ -260,7 +260,7 @@ pub fn export_chats(
     }
 
     // Sort by message count descending
-    exported_chats.sort_by(|a, b| b.messages.len().cmp(&a.messages.len()));
+    exported_chats.sort_by_key(|c| std::cmp::Reverse(c.messages.len()));
 
     // Write each chat to a separate JSON file and create zip
     let zip_path = temp_dir.path().join("export.zip");

src-tauri/src/lib.rs

@@ -196,7 +196,7 @@ pub fn list_chats(custom_db_path: Option<&std::path::Path>) -> Result<Vec<ChatIn
         .collect();
 
     // Sort by last message date descending (most recent first)
-    result.sort_by(|a, b| b.1.cmp(&a.1));
+    result.sort_by_key(|item| std::cmp::Reverse(item.1));
 
     // Extract just the ChatInfo
     let result: Vec<ChatInfo> = result.into_iter().map(|(info, _)| info).collect();