refactor: remove legacy Thymeleaf web UI controllers and templates (#5406)

# Description of Changes

This pull request completes the removal of Thymeleaf template engine
support and documentation from the Stirling-PDF codebase and developer
guides, reflecting the project's full migration to a React-based
frontend. It removes all references to Thymeleaf in code, configuration,
and documentation, and updates guides to focus exclusively on the React
SPA architecture. This streamlines the codebase and clarifies the
development workflow for contributors.

**Codebase cleanup: Thymeleaf removal**
- Removed all commented-out Thymeleaf dependencies and related
configuration beans from `build.gradle` and `AppConfig.java`
(`app/common/build.gradle`,
`app/common/src/main/java/stirling/software/common/configuration/AppConfig.java`)
[[1]](diffhunk://#diff-2a1a21726f33b05d16451237c68d6df91a5f4a58419d839715f3f1538a9a14aeL32)
[[2]](diffhunk://#diff-70792df9a0ab5675ded888c9eb8e2815c780d7b39f4bda8cf2da51d1b336899aL67-L76).
- Fully commented out (as a precursor to future deletion) the
`FileFallbackTemplateResolver` and `InputStreamTemplateResource`
classes, which were only used for Thymeleaf template resolution
(`app/common/src/main/java/stirling/software/common/configuration/FileFallbackTemplateResolver.java`,
`app/common/src/main/java/stirling/software/common/model/InputStreamTemplateResource.java`)
[[1]](diffhunk://#diff-e2bc7614074316b972355cb7dda47b98f75b00eb6b2ca4f143a680ab2803dcd8L1-L49)
[[2]](diffhunk://#diff-ab10ee12d8de8fb77759e931170373d388bde04bad6d0e42a0ab674355ef7ef3L1-L40).

**Documentation updates: React-only focus**
- Removed all instructions and references to migrating or developing
with Thymeleaf templates in `DeveloperGuide.md` and `ADDING_TOOLS.md`,
including detailed Thymeleaf usage examples, migration steps, and
translation key usage in templates
[[1]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL5-R5)
[[2]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL41-L43)
[[3]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL103-L105)
[[4]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL157)
[[5]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL312)
[[6]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL404-R396)
[[7]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL451-L505)
[[8]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL530-R467)
[[9]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL585-L669)
[[10]](diffhunk://#diff-ccd22fcbec8148152c8c77b85fbfe2633a6707b5ad50c2ef88fa87e2c47ea88fL699-L709)
[[11]](diffhunk://#diff-e2f8148ea620602b7761e8ee24afeac1c577476630528e210fe0b22e950016ddL3-R3)
[[12]](diffhunk://#diff-e2f8148ea620602b7761e8ee24afeac1c577476630528e210fe0b22e950016ddL267-R267).
- Updated architecture descriptions in `CLAUDE.md` to reflect that the
frontend is now exclusively a React SPA and that Thymeleaf templates
have been fully replaced
[[1]](diffhunk://#diff-6ebdb617a8104a7756d0cf36578ab01103dc9f07e4dc6feb751296b9c402faf7L131-R132)
[[2]](diffhunk://#diff-6ebdb617a8104a7756d0cf36578ab01103dc9f07e4dc6feb751296b9c402faf7L143-L144).

**Labeler configuration update**
- Removed labeler rules for files related to the old Thymeleaf-based web
controllers and UI directories, as these are now obsolete
(`.github/labeler-config-srvaroa.yml`).

These changes ensure the codebase and documentation are consistent with
the new React-only frontend approach, reducing maintenance overhead and
potential confusion for contributors.

---

## 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.

CLAUDE.md

@@ -128,8 +128,8 @@ return useToolOperation({
 ## Architecture Overview
 
 ### Project Structure
-- **Backend**: Spring Boot application with Thymeleaf templating
-- **Frontend**: React-based SPA in `/frontend` directory (Thymeleaf templates fully replaced)
+- **Backend**: Spring Boot application
+- **Frontend**: React-based SPA in `/frontend` directory
   - **File Storage**: IndexedDB for client-side file persistence and thumbnails
   - **Internationalization**: JSON-based translations (converted from backend .properties)
 - **PDF Processing**: PDFBox for core PDF operations, LibreOffice for conversions, PDF.js for client-side rendering
@@ -140,8 +140,6 @@ return useToolOperation({
 - **API Controllers** (`src/main/java/.../controller/api/`): REST endpoints for PDF operations
   - Organized by function: converters, security, misc, pipeline
   - Follow pattern: `@RestController` + `@RequestMapping("/api/v1/...")`
-- **Web Controllers** (`src/main/java/.../controller/web/`): Serve Thymeleaf templates
-  - Pattern: `@Controller` + return template names
 
 ### Key Components
 - **SPDFApplication.java**: Main application class with desktop UI and browser launching logic