refactor(ssrf): default enum MEDIUM prevents OFF=false (#4280)

# Description of Changes

- **What was changed**
  - **URL to PDF flow**
- Changed `ConvertWebsiteToPDF#urlToPdf` to return `ResponseEntity<?>`
and perform a redirect (`303 SEE_OTHER`) back to `/url-to-pdf` with an
`error` query param instead of throwing exceptions.
- Added alert rendering in `url-to-pdf.html` using `param.error` for
localized error display.
- Introduced new translation key `error.invalidUrlFormat` in
`messages_en_GB.properties`.
  - **Security / SSRF**
- Migrated `ApplicationProperties.System.UrlSecurity.level` from
`String` to `SsrfProtectionLevel` enum.
- Default now set to `SsrfProtectionLevel.MEDIUM` (`// MAX, MEDIUM,
OFF`).
- This avoids the issue where setting `OFF` returned `false` in
configuration parsing.
- Updated `SsrfProtectionService#parseProtectionLevel` accordingly
(using `level.name()`).
  - **Repo hygiene**
    - Added `**/LOCAL_APPDATA_FONTCONFIG_CACHE/**` to `.gitignore`.

- **Why the change was made**
- Provide user-friendly, localized error messages instead of exposing
internal exceptions on URL-to-PDF conversions.
- Ensure SSRF protection level parsing is type-safe and consistent—`OFF`
can now be set without yielding a misleading `false` state.
  - Prevent unwanted fontconfig cache files from being tracked in Git.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] 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)
- [x] I have performed a self-review of my own code
- [x] 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)

### 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.

app/core/src/main/java/stirling/software/SPDF/controller/api/converters/ConvertWebsiteToPDF.java

@@ -1,17 +1,21 @@
 package stirling.software.SPDF.controller.api.converters;
 
 import java.io.IOException;
+import java.net.URI;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
 
 import org.apache.pdfbox.pdmodel.PDDocument;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.servlet.support.ServletUriComponentsBuilder;
+import org.springframework.web.util.UriComponentsBuilder;
 
 import io.swagger.v3.oas.annotations.Operation;
 import io.swagger.v3.oas.annotations.tags.Tag;
@@ -23,7 +27,6 @@ import stirling.software.SPDF.model.api.converters.UrlToPdfRequest;
 import stirling.software.common.configuration.RuntimePathConfig;
 import stirling.software.common.model.ApplicationProperties;
 import stirling.software.common.service.CustomPDFDocumentFactory;
-import stirling.software.common.util.ExceptionUtils;
 import stirling.software.common.util.GeneralUtils;
 import stirling.software.common.util.ProcessExecutor;
 import stirling.software.common.util.ProcessExecutor.ProcessExecutorResult;
@@ -46,24 +49,43 @@ public class ConvertWebsiteToPDF {
             description =
                     "This endpoint fetches content from a URL and converts it to a PDF format."
                             + " Input:N/A Output:PDF Type:SISO")
-    public ResponseEntity<byte[]> urlToPdf(@ModelAttribute UrlToPdfRequest request)
+    public ResponseEntity<?> urlToPdf(@ModelAttribute UrlToPdfRequest request)
             throws IOException, InterruptedException {
         String URL = request.getUrlInput();
+        UriComponentsBuilder uriComponentsBuilder =
+                ServletUriComponentsBuilder.fromCurrentContextPath().path("/url-to-pdf");
+        URI location = null;
+        HttpStatus status = HttpStatus.SEE_OTHER;
 
         if (!applicationProperties.getSystem().getEnableUrlToPDF()) {
-            throw ExceptionUtils.createIllegalArgumentException(
-                    "error.endpointDisabled", "This endpoint has been disabled by the admin");
-        }
+            location =
+                    uriComponentsBuilder
+                            .queryParam("error", "error.endpointDisabled")
+                            .build()
+                            .toUri();
+        } else
+
         // Validate the URL format
         if (!URL.matches("^https?://.*") || !GeneralUtils.isValidURL(URL)) {
-            throw ExceptionUtils.createInvalidArgumentException(
-                    "URL", "provided format is invalid");
-        }
+            location =
+                    uriComponentsBuilder
+                            .queryParam("error", "error.invalidUrlFormat")
+                            .build()
+                            .toUri();
+        } else
 
         // validate the URL is reachable
         if (!GeneralUtils.isURLReachable(URL)) {
-            throw ExceptionUtils.createIllegalArgumentException(
-                    "error.urlNotReachable", "URL is not reachable, please provide a valid URL");
+            location =
+                    uriComponentsBuilder
+                            .queryParam("error", "error.urlNotReachable")
+                            .build()
+                            .toUri();
+        }
+
+        if (location != null) {
+            log.info("Redirecting to: {}", location.toString());
+            return ResponseEntity.status(status).location(location).build();
         }
 
         Path tempOutputFile = null;