chore: upgrade Gradle to 9.6.1 across CI, Docker, and wrapper (#6892)

# Description of Changes

This change upgrades the project from Gradle **9.6.0** to **9.6.1**
across all build environments to keep the toolchain consistent and
aligned.

### What was changed

- Updated the Gradle Wrapper to **9.6.1**.
- Updated all GitHub Actions workflows using
`gradle/actions/setup-gradle` to install Gradle **9.6.1**.
- Updated all Docker build stages to use the `gradle:9.6.1-jdk25` image
with the corresponding pinned image digest.
- Regenerated the Windows Gradle wrapper script, resulting in minor
comment updates (`Gradle` → `gradlew`).

### Why the change was made

- Keep the project up to date with the latest Gradle patch release.
- Ensure all local, CI, and Docker build environments use the same
Gradle version.
- Benefit from the latest bug fixes and maintenance improvements
included in Gradle 9.6.1.

---

## 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/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 run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

docker/backend/Dockerfile

@@ -4,7 +4,7 @@ ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application (backend only, no frontend)
-FROM gradle:9.6.0-jdk25@sha256:e3905233ae349e72016daf8a0e19f085a1dd89ded8ec88b3d8335d3fd0b350f4 AS app-build
+FROM gradle:9.6.1-jdk25@sha256:d25357611c559203299f3c6673101154156bdd28985858ff6f465fc69bd27ca4 AS app-build
 
 # JDK 25+: --add-exports is no longer accepted via JAVA_TOOL_OPTIONS; use JDK_JAVA_OPTIONS instead
 ENV JDK_JAVA_OPTIONS="--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \

docker/embedded/Dockerfile

@@ -5,7 +5,7 @@ ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application and frontend
-FROM gradle:9.6.0-jdk25@sha256:e3905233ae349e72016daf8a0e19f085a1dd89ded8ec88b3d8335d3fd0b350f4 AS app-build
+FROM gradle:9.6.1-jdk25@sha256:d25357611c559203299f3c6673101154156bdd28985858ff6f465fc69bd27ca4 AS app-build
 
 ARG TASK_VERSION=3.49.1
 RUN apt-get update \

docker/embedded/Dockerfile.fat

@@ -6,7 +6,7 @@ ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application and frontend
-FROM gradle:9.6.0-jdk25@sha256:e3905233ae349e72016daf8a0e19f085a1dd89ded8ec88b3d8335d3fd0b350f4 AS app-build
+FROM gradle:9.6.1-jdk25@sha256:d25357611c559203299f3c6673101154156bdd28985858ff6f465fc69bd27ca4 AS app-build
 
 ARG TASK_VERSION=3.49.1
 RUN apt-get update \

docker/embedded/Dockerfile.ultra-lite

@@ -2,7 +2,7 @@
 # Single JAR contains both frontend and backend with minimal dependencies
 
 # Stage 1: Build application with embedded frontend
-FROM gradle:9.6.0-jdk25@sha256:e3905233ae349e72016daf8a0e19f085a1dd89ded8ec88b3d8335d3fd0b350f4 AS build
+FROM gradle:9.6.1-jdk25@sha256:d25357611c559203299f3c6673101154156bdd28985858ff6f465fc69bd27ca4 AS build
 
 # Install Node.js and npm for frontend build
 ARG TASK_VERSION=3.49.1