fix(api): return JSON responses for admin settings + API key endpoints to prevent Tauri client parse errors (#5437)

# Description of Changes

```console
index-DsORDqQQ.js:124 \n [TauriHttpClient] Network error: \n{url: 'http://localhost:8080/api/v1/admin/settings', method: 'PUT', errorType: 'ERR_NETWORK', originalMessage: `Failed to execute 'close' on 'ReadableStreamDefaul…cted token 'S', "Successful"... is not valid JSON`, stack: `SyntaxError: Unexpected token 'S', "Successful"...…lback (<anonymous>:284:7)\n    at <anonymous>:1:28`}\nerrorType\n: \n"ERR_NETWORK"\nmethod\n: \n"PUT"\noriginalMessage\n: \n"Failed to execute 'close' on 'ReadableStreamDefaultController': Unexpected token 'S', \"Successful\"... is not valid JSON"\nstack\n: \n"SyntaxError: Unexpected token 'S', \"Successful\"... is not valid JSON\n    at A.onmessage (http://tauri.localhost/assets/index-DsORDqQQ.js:124:22714)\n    at http://tauri.localhost/assets/index-DsORDqQQ.js:124:20748\n    at <anonymous>:272:26\n    at Object.runCallback (<anonymous>:284:7)\n    at <anonymous>:1:28"\nurl\n: \n"http://localhost:8080/api/v1/admin/settings"

index-DXbk7lbS.js:124 \n [TauriHttpClient] Network error: \n{url: 'http://localhost:8080/api/v1/user/get-api-key', method: 'POST', errorType: 'ERR_NETWORK', originalMessage: `Failed to execute 'close' on 'ReadableStreamDefaul…cted token 'a', "a72f6b26-1"... is not valid JSON`, stack: `SyntaxError: Unexpected token 'a', "a72f6b26-1"...…lback (<anonymous>:284:7)\n    at <anonymous>:1:28`}\nerrorType\n: \n"ERR_NETWORK"\nmethod\n: \n"POST"\noriginalMessage\n: \n"Failed to execute 'close' on 'ReadableStreamDefaultController': Unexpected token 'a', \"a72f6b26-1\"... is not valid JSON"\nstack\n: \n"SyntaxError: Unexpected token 'a', \"a72f6b26-1\"... is not valid JSON\n    at A.onmessage (http://tauri.localhost/assets/index-DXbk7lbS.js:124:22714)\n    at http://tauri.localhost/assets/index-DXbk7lbS.js:124:20748\n    at <anonymous>:272:26\n    at Object.runCallback (<anonymous>:284:7)\n    at <anonymous>:1:28"\nurl\n: \n"http://localhost:8080/api/v1/user/get-api-key"
```


This pull request fixes a self-hosting issue where the Tauri HTTP client
fails with `Unexpected token ... is not valid JSON` because certain API
endpoints returned plain text responses.

## What was changed

- Updated `AdminSettingsController`:
- Changed `updateSettings` and `updateSettingsSection` to return
structured JSON objects instead of raw strings.
- Standardized success and error payloads using a `Map<String, Object>`
with keys like `message` and `error`.
- Updated `UserController`:
- Changed `/api/v1/user/get-api-key` and `/api/v1/user/update-api-key`
to return JSON objects (`{ "apiKey": "..." }`) and JSON error objects
instead of plain text.

## Why the change was made

- The Tauri client expects JSON responses and attempts to parse them.
Returning plain strings like `"Successful..."` or an API key string
causes JSON parsing to fail, resulting in network errors on self-hosted
setups.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: James Brunton <[email protected]>

frontend/public/locales/en-GB/translation.toml

@@ -5404,6 +5404,15 @@ impact = "Any applications or services currently using these keys will stop work
 confirmPrompt = "Are you sure you want to continue?"
 confirmCta = "Refresh Keys"
 
+[config.apiKeys.alert]
+apiKeyErrorTitle = "API Key Error"
+failedToCreateApiKey = "Failed to create API key."
+failedToRetrieveApiKey = "Failed to retrieve API key from response."
+failedToFetchApiKey = "Failed to fetch API key."
+apiKeyRefreshed = "API Key Refreshed"
+apiKeyRefreshedBody = "Your API key has been successfully refreshed."
+failedToRefreshApiKey = "Failed to refresh API key."
+
 [AddAttachmentsRequest]
 attachments = "Select Attachments"
 info = "Select files to attach to your PDF. These files will be embedded and accessible through the PDF's attachment panel."