refactor(codex): unify custom model_provider routing key to "custom"

- Always emit `model_provider = "custom"` from deep link, UniversalProvider, and the universal form modal so future writes share one stable routing key.
- Add `codex_provider_template_v1` local migration that rewrites legacy keys (aihubmix/ccswitch/...) under `[model_providers.custom]`, updates profile refs, and backs up the original settings_config under `~/.cc-switch/backups/<timestamp>/providers/`.
- Tighten history migration source detection to a whitelist plus `[model_providers.<id>]` existence check so user-authored keys are never rewritten in jsonl/state DB.
- Encode deep link name/model/endpoint through `toml_edit::Value` so display names containing quotes or backslashes no longer break the generated config.toml.
- Stabilize provider settings backup filename hash with Sha256 (was process-random SipHash).

src-tauri/src/commands/settings.rs

@@ -34,6 +34,10 @@ fn merge_settings_for_save(
                 .codex_third_party_history_provider_bucket_v1
                 .clone();
         }
+        if incoming_migrations.codex_provider_template_v1.is_none() {
+            incoming_migrations.codex_provider_template_v1 =
+                existing_migrations.codex_provider_template_v1.clone();
+        }
     }
     incoming
 }
@@ -98,8 +102,8 @@ pub async fn set_auto_launch(enabled: bool) -> Result<bool, String> {
 mod tests {
     use super::merge_settings_for_save;
     use crate::settings::{
-        AppSettings, CodexThirdPartyHistoryProviderBucketMigration, LocalMigrations,
-        WebDavSyncSettings,
+        AppSettings, CodexProviderTemplateMigration, CodexThirdPartyHistoryProviderBucketMigration,
+        LocalMigrations, WebDavSyncSettings,
     };
 
     #[test]
@@ -236,6 +240,10 @@ mod tests {
                         scanned_history_files: true,
                     },
                 ),
+                codex_provider_template_v1: Some(CodexProviderTemplateMigration {
+                    completed_at: "2026-05-20T00:01:00Z".to_string(),
+                    migrated_provider_ids: vec!["legacy".to_string()],
+                }),
             }),
             ..AppSettings::default()
         };
@@ -255,6 +263,16 @@ mod tests {
         assert_eq!(migration.target_provider_id, "custom");
         assert_eq!(migration.migrated_jsonl_files, 2);
         assert_eq!(migration.migrated_state_rows, 3);
+
+        let template_migration = merged
+            .local_migrations
+            .as_ref()
+            .and_then(|migrations| migrations.codex_provider_template_v1.as_ref())
+            .expect("template migration marker should be preserved");
+        assert_eq!(
+            template_migration.migrated_provider_ids,
+            vec!["legacy".to_string()]
+        );
     }
 }