fix(core): reinstall frontend dependencies when iconify package is missing (#5432)

# Description of Changes

### What was changed
- Updated the Gradle `npmInstall` task to detect a missing required
devDependency (`@iconify-json/material-symbols`) even when
`node_modules` exists.
- If the package is missing, the task will no longer skip `npm install`;
it will force a reinstall instead.

### Why the change was made
- The frontend build (`npm run generate-icons`) can fail with `Cannot
find module '@iconify-json/material-symbols'` when `node_modules` is
present but incomplete/corrupted.
- The current "node_modules exists => skip install" shortcut is not
sufficient for this scenario, causing Gradle builds to fail during
`npmBuild`.

---

## 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.

app/core/build.gradle

@@ -208,6 +208,14 @@ tasks.register('npmInstall', Exec) {
             println "node_modules not found, will install..."
             return true
         }
+
+        // if required devDependency is missing, reinstall
+        def iconifyPkg = new File(frontendDir, 'node_modules/@iconify-json/material-symbols/package.json')
+        if (!iconifyPkg.exists()) {
+            println "@iconify-json/material-symbols missing, will reinstall..."
+            return true
+        }
+
         def packageJson = new File(frontendDir, 'package.json')
         def packageLock = new File(frontendDir, 'package-lock.json')
         def isOutdated = nodeModules.lastModified() < packageJson.lastModified() ||