Deprecate integrated Prettier support

[pplancq]

Type of Pull Request:

• Bug fix
• New feature
• Documentation
• Other (deprecation)

Associated Issue:
Related to #1848

Context:
This PR prepares the separation of formatting responsibilities described in issue #1848. Prettier should run independently from ESLint and Stylelint so that those tools focus on code quality and avoid unnecessary formatting work. This is the non-breaking deprecation phase; the actual removal will be handled separately.

Proposed Changes:

• Deprecate the enablePrettier: "on" option in @pplancq/eslint-config and emit a warning when it is used.
• Update the ESLint configuration documentation to recommend running Prettier separately.
• Add guidance for using eslint-config-prettier to disable conflicting formatting rules.
• Update package ESLint configurations to use eslint-config-prettier instead of the integrated Prettier ESLint plugin where appropriate.
• Update package scripts and lint-staged configuration to run Prettier independently.
• Update the React template’s ESLint, Stylelint, Prettier, testing, and build-related configuration files to follow the new workflow.
• Update dependencies and lockfile entries accordingly.

Checklist:

• I have verified that my changes work as expected
• I have updated the documentation if necessary
• I have thought to rebase my branch
• I have applied the correct label according to the type of PR (bug/feature/documentation)

Additional Information:
This PR is intentionally non-breaking and does not close issue #1848. It represents the first deprecation phase only; removal of the deprecated Prettier integrations and the related major-version changes should be addressed in a subsequent PR.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud