Refactor IPC and DTO structures; remove unused code and streamline message handling

- Removed unused structs and methods from `response.rs`, `usage.rs`, and `client.rs`.
- Simplified `Connection` handling in `conn.rs` to only support Unix sockets.
- Updated `IpcServer` to exclusively use Unix sockets and removed TCP handling.
- Cleaned up `editlog.rs` by removing loading and recent entry methods.
- Refactored `memory.rs` to eliminate unused functions related to lesson promotion and retrospective creation.
- Enhanced `search.rs` to support multiple search providers and improved error handling.
- Updated chat view logic to simplify message display and improve user experience.
- Removed deprecated modules and constants from various files to streamline the codebase.

src/app/runtime/commands.rs

@@ -41,10 +41,7 @@ pub fn apply_command(command: Command) -> Vec<Action> {
             }]
         }
         Command::Login { provider } => {
-            vec![Action::SystemNote {
-                kind: "oauth".to_string(),
-                message: format!("OAuth login flow started for {}", provider),
-            }]
+            vec![Action::StartOAuth { provider }]
         }
         Command::Unknown(cmd) => {
             vec![Action::SystemNote {