fix(chat): preserve Gemini CLI history sessions on reopen

When reopening a Gemini CLI history session, session/load fails with
"Authentication required" and the fallback session/new overwrites the
DB external_id with a new session ID that has no corresponding file,
causing all historical messages to disappear.

- Skip session/new when session/load returns "Authentication required"
- Add Gemini to the parser fallback so stale external_ids recover via
  folder_path + started_at matching
- Guard externalIdSavedRef for existing conversations to prevent
  session/new from overwriting the persisted external_id
- Only update conversation status on disconnect when user has sent a
  message, avoiding spurious "completed" flips on pure history views

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/acp/connection.rs

@@ -844,6 +844,12 @@ async fn run_connection(
                         // Only emit a visible error for unexpected failures;
                         // "Method not found" is expected for agents that don't
                         // support session resume (e.g. Cline).
+                        // "Authentication required" is expected for agents whose
+                        // credentials have expired (e.g. Gemini CLI) — skip
+                        // session/new too since it will also fail.
+                        if err_str.contains("Authentication required") {
+                            return Ok(());
+                        }
                         if !err_str.contains("Method not found") {
                             crate::web::event_bridge::emit_event(
                                 &emitter_clone,