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

[Ludy87]

Description of Changes

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
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (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 (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

Translations (if applicable)

• I ran scripts/counter_translation.py

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 for more details.