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/resources/messages_en_GB.properties

@@ -193,6 +193,7 @@ error.fileFormatRequired=File must be in {0} format
 error.invalidFormat=Invalid {0} format: {1}
 error.endpointDisabled=This endpoint has been disabled by the admin
 error.urlNotReachable=URL is not reachable, please provide a valid URL
+error.invalidUrlFormat=Invalid URL format provided. The provided format is invalid.
 
 # DPI and image rendering messages - used by frontend for dynamic translation
 # Backend sends: [TRANSLATE:messageKey:arg1,arg2] English message

app/core/src/main/resources/templates/convert/url-to-pdf.html

@@ -19,6 +19,7 @@
               <span class="material-symbols-rounded tool-header-icon convertto">link</span>
               <span class="tool-header-text" th:text="#{URLToPDF.header}"></span>
             </div>
+            <p th:if="${not #lists.isEmpty(param.error)}" th:text="#{${param.error[0]}}" class="alert alert-danger text-center"></p>
             <form method="post" enctype="multipart/form-data" th:action="@{'/api/v1/convert/url/pdf'}">
               <input type="text" class="form-control" id="urlInput" name="urlInput" placeholder="http://">
               <br>