feat: add user secrets management panel (#8473)

## Summary

Add a Secrets panel to user settings for managing third-party API keys
(HuggingFace, Civitai). Secrets are encrypted server-side; plaintext
values are never returned.

## Changes

- Add `SecretsPanel` to settings for managing third-party API keys
- Create `secretsApi` service following the `workspaceApi` pattern
- Add `SecretListItem` and `SecretFormDialog` components
- Add `user_secrets_enabled` feature flag
- Support HuggingFace and Civitai providers
- Add i18n translations for secrets UI

## Files Added

- `src/platform/secrets/types.ts` - TypeScript types
- `src/platform/secrets/api/secretsApi.ts` - Axios-based API service
- `src/platform/secrets/components/SecretsPanel.vue` - Main settings
panel
- `src/platform/secrets/components/SecretListItem.vue` - Individual
secret row
- `src/platform/secrets/components/SecretFormDialog.vue` - Create/edit
dialog

## Files Modified

- `src/platform/remoteConfig/types.ts` - Add `user_secrets_enabled` flag
type
- `src/composables/useFeatureFlags.ts` - Add flag getter
- `src/platform/settings/composables/useSettingUI.ts` - Integrate
secrets panel
- `src/locales/en/main.json` - Add translations

## Testing

Panel appears in Settings under:
- "Workspace" group when team workspaces is enabled
- "Account" group in legacy mode

Only visible when user is logged in AND `user_secrets_enabled` feature
flag is enabled.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8473-feat-add-user-secrets-management-panel-2f86d73d36508187b4a1ed04ce07ce51)
by [Unito](https://www.unito.io)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Secrets management UI in Settings with create/edit/delete for API
keys/secrets; settings dialog entry and contextual Secrets hint in
upload/import flows (feature-flag gated).
* **APIs**
* Added backend-facing secrets CRUD surface and client-side
form/composable support for managing secrets.
* **Localization**
* New English translations for Secrets UI and many expanded asset
import/upload error and hint messages.
* **Tests**
* Comprehensive unit tests for secrets UI, form flows, and composables.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Amp <amp@ampcode.com>

src/composables/useFeatureFlags.ts

@@ -22,7 +22,8 @@ export enum ServerFeatureFlag {
   HUGGINGFACE_MODEL_IMPORT_ENABLED = 'huggingface_model_import_enabled',
   LINEAR_TOGGLE_ENABLED = 'linear_toggle_enabled',
   ASYNC_MODEL_UPLOAD_ENABLED = 'async_model_upload_enabled',
-  TEAM_WORKSPACES_ENABLED = 'team_workspaces_enabled'
+  TEAM_WORKSPACES_ENABLED = 'team_workspaces_enabled',
+  USER_SECRETS_ENABLED = 'user_secrets_enabled'
 }
 
 /**
@@ -116,6 +117,12 @@ export function useFeatureFlags() {
         remoteConfig.value.team_workspaces_enabled ??
         api.getServerFeature(ServerFeatureFlag.TEAM_WORKSPACES_ENABLED, false)
       )
+    },
+    get userSecretsEnabled() {
+      return (
+        remoteConfig.value.user_secrets_enabled ??
+        api.getServerFeature(ServerFeatureFlag.USER_SECRETS_ENABLED, false)
+      )
     }
   })