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/components/universal/UniversalProviderFormModal.tsx

@@ -152,12 +152,12 @@ export function UniversalProviderFormModal({
     const codexBaseUrl = baseUrl.endsWith("/v1")
       ? baseUrl
       : `${baseUrl.replace(/\/+$/, "")}/v1`;
-    const configToml = `model_provider = "newapi"
+    const configToml = `model_provider = "custom"
 model = "${model}"
 model_reasoning_effort = "${reasoningEffort}"
 disable_response_storage = true
 
-[model_providers.newapi]
+[model_providers.custom]
 name = "NewAPI"
 base_url = "${codexBaseUrl}"
 wire_api = "responses"