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.

ADDING_TOOLS.md

@@ -1,6 +1,6 @@
 # Adding New React Tools to Stirling PDF
 
-This guide covers how to add new PDF tools to the React frontend, either by migrating existing Thymeleaf templates or creating entirely new tools.
+This guide covers how to add new PDF tools to the React frontend.
 
 ## Overview
 
@@ -188,7 +188,7 @@ import { use[ToolName]Tips } from "../components/tooltips/use[ToolName]Tips";
 
 const [ToolName] = (props: BaseToolProps) => {
   const tips = use[ToolName]Tips();
-  
+
   // In your steps array:
   steps: [
     {
@@ -257,22 +257,14 @@ Update translation files. **Important: Only update `en-GB` files** - other langu
 - Add `options.*` keys if your tool has settings with descriptions
 
 **Tooltip Writing Guidelines:**
-- **Use simple, everyday language** - avoid technical terms like "converts interactive elements" 
+- **Use simple, everyday language** - avoid technical terms like "converts interactive elements"
 - **Focus on benefits** - explain what the user gains, not how it works internally
 - **Use concrete examples** - "text boxes become regular text" vs "form fields are flattened"
 - **Answer user questions** - "What does this do?", "When should I use this?", "What's this option for?"
 - **Keep descriptions concise** - 1-2 sentences maximum per section
 - **Use bullet points** for multiple benefits or features
 
-## 6. Migration from Thymeleaf
-When migrating existing Thymeleaf templates:
-
-1. **Identify Form Parameters**: Look at the original `<form>` inputs to determine parameter structure
-2. **Extract Translation Keys**: Find `#{key.name}` references and add them to JSON translations (For many tools these translations will already exist but some parts will be missing)
-3. **Map API Endpoint**: Note the `th:action` URL for the operation hook
-4. **Preserve Functionality**: Ensure all original form behaviour is replicated which is applicable to V2 react UI
-
-## 7. Testing Your Tool
+## 6. Testing Your Tool
 - Verify tool appears in UI with correct icon and description
 - Test with various file sizes and types
 - Confirm translations work