Added Slovenian translation

[thehijacker]

📝 Description

Added support for my native language. Everyone in our family will use Booklore and kids are not good at English yet.

There are no guidlines about translating so I hope copying json files from booklore/booklore-ui/src/i18n/en to booklore/booklore-ui/src/i18n/sl and translating every string is enough.

🏷️ Type of Change

• Bug fix
• New feature
• Enhancement to existing feature
• Refactor (no behavior change)
• Breaking change (existing functionality affected)
• Documentation update

🔧 Changes

• Added Slovenian translation files

🧪 Testing (MANDATORY)

I validated all json files. But have not tested anything as I will need to wait for next Booklore build to see the new strings.

Regression testing:

• Tested all json to confirm they are valid.

Test output:

📸 Screen Recording / Screenshots (MANDATORY)

✅ Pre-Submission Checklist

All boxes must be checked before requesting review. Incomplete PRs will be closed without review. No exceptions.

• This PR is linked to an approved issue
• Code follows project style guidelines and conventions
• Branch is up to date with develop (merge conflicts resolved)
• I ran the full stack locally (backend + frontend + database) and verified the change works
• Automated tests added or updated to cover changes (backend and frontend)
• All tests pass locally and output is pasted above
• Screen recording or screenshots are attached above proving the change works
• PR is a single focused change (one bug fix OR one feature, not multiple unrelated changes)
• PR is reasonably scoped (PRs over 1000+ changed lines will be closed, split into smaller PRs)
• No unsolicited refactors, cleanups, or "improvements" are bundled in
• Flyway migration versioning is correct (if schema was modified)
• Documentation PR submitted to booklore-docs (if user-facing changes)

🤖 AI-Assisted Contributions

If any part of this PR was generated or assisted by AI tools (Copilot, Claude, ChatGPT, etc.), all items below are mandatory. You are fully responsible for every line you submit. "The AI wrote it" is not an excuse, and AI-generated PRs that clearly haven't been reviewed are the #1 reason PRs get closed.

• I have read and understand every line of this PR and can explain any part of it during review
• I personally ran the code and verified it works (not just trusted the AI's output)
• PR is scoped to a single logical change, not a dump of everything the AI suggested
• Tests validate actual behavior, not just coverage (AI-generated tests often assert nothing meaningful)
• No dead code, placeholder comments, TODOs, or unused scaffolding left behind by AI
• I did not submit refactors, style changes, or "improvements" the AI suggested beyond the scope of the issue

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[thehijacker]

I have uploaded the files to Weblate and will finish translation there.