feat: unify Codex third-party providers into stable "custom" history bucket

Codex filters resume history by `model_provider`, so switching between
provider-specific ids like `rightcode` and `aihubmix` made past sessions
appear to vanish. Collapse all third-party providers into a single
stable bucket so cross-switch history stays visible.

- Normalize live `model_provider` to "custom" on every Codex write
  (reserved built-in ids like openai/ollama are preserved).
- Add device-level one-shot migration that rewrites historical JSONL
  session files and the `state_5.sqlite` threads table from legacy
  provider ids into the "custom" bucket. Backs up originals under
  `~/.cc-switch/backups/codex-history-provider-migration-v1/` and uses
  the SQLite Backup API for the state DB.
- Record completion in `settings.json` under `localMigrations` so the
  migration is strictly idempotent across launches.
- Update Codex provider preset templates to emit `model_provider = "custom"`
  out of the box.

src-tauri/src/commands/settings.rs

@@ -21,6 +21,20 @@ fn merge_settings_for_save(
         }
         _ => {}
     }
+    if incoming.local_migrations.is_none() {
+        incoming.local_migrations = existing.local_migrations.clone();
+    } else if let (Some(incoming_migrations), Some(existing_migrations)) =
+        (&mut incoming.local_migrations, &existing.local_migrations)
+    {
+        if incoming_migrations
+            .codex_third_party_history_provider_bucket_v1
+            .is_none()
+        {
+            incoming_migrations.codex_third_party_history_provider_bucket_v1 = existing_migrations
+                .codex_third_party_history_provider_bucket_v1
+                .clone();
+        }
+    }
     incoming
 }
 
@@ -83,7 +97,10 @@ pub async fn set_auto_launch(enabled: bool) -> Result<bool, String> {
 #[cfg(test)]
 mod tests {
     use super::merge_settings_for_save;
-    use crate::settings::{AppSettings, WebDavSyncSettings};
+    use crate::settings::{
+        AppSettings, CodexThirdPartyHistoryProviderBucketMigration, LocalMigrations,
+        WebDavSyncSettings,
+    };
 
     #[test]
     fn save_settings_should_preserve_existing_webdav_when_payload_omits_it() {
@@ -204,6 +221,40 @@ mod tests {
             Some("")
         );
     }
+
+    #[test]
+    fn save_settings_should_preserve_local_migrations_when_payload_omits_it() {
+        let existing = AppSettings {
+            local_migrations: Some(LocalMigrations {
+                codex_third_party_history_provider_bucket_v1: Some(
+                    CodexThirdPartyHistoryProviderBucketMigration {
+                        completed_at: "2026-05-20T00:00:00Z".to_string(),
+                        target_provider_id: "custom".to_string(),
+                        source_provider_ids: vec!["rightcode".to_string()],
+                        migrated_jsonl_files: 2,
+                        migrated_state_rows: 3,
+                    },
+                ),
+            }),
+            ..AppSettings::default()
+        };
+
+        let incoming = AppSettings::default();
+        let merged = merge_settings_for_save(incoming, &existing);
+
+        let migration = merged
+            .local_migrations
+            .as_ref()
+            .and_then(|migrations| {
+                migrations
+                    .codex_third_party_history_provider_bucket_v1
+                    .as_ref()
+            })
+            .expect("local migration marker should be preserved");
+        assert_eq!(migration.target_provider_id, "custom");
+        assert_eq!(migration.migrated_jsonl_files, 2);
+        assert_eq!(migration.migrated_state_rows, 3);
+    }
 }
 
 /// 获取开机自启状态