fix(ui): reload shelves when enabling Kobo sync

[acx10]

📝 Description

Fixes #2528

When enabling Kobo sync, the backend creates a "Kobo" shelf but the UI didn't refresh the shelf list — the user had to manually reload the page to see it. Disabling sync correctly removed the shelf without a refresh because reloadShelves() was only called in the disable path.

🏷️ Type of Change

• Bug fix

🔧 Changes

• Removed the !syncEnabled guard around shelfService.reloadShelves() in updateKoboSettings() so shelves reload on both enable and disable

🧪 Testing

• Enable Kobo sync → Kobo shelf appears in sidebar immediately without page refresh
• Disable Kobo sync → Kobo shelf is removed from sidebar immediately (existing behavior preserved)

📸 Screenshots / Video (MANDATORY)

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✅ Pre-Submission Checklist

• Code follows project style guidelines and conventions
• Branch is up to date with develop (merge conflicts resolved)
• Automated tests added or updated to cover changes (backend and frontend)
• All tests pass locally (./gradlew test for backend, ng test for frontend)
• Changes manually verified in local dev environment (including related features)
• Screenshots or video proving the change works are attached above (MANDATORY)
• Flyway migration versioning is correct (if schema was modified)
• Documentation PR submitted to booklore-docs (if user-facing changes)
• PR is reasonably scoped (PRs over 1000+ changed lines will be closed, split into smaller PRs)

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💬 Additional Context (optional)

The reloadShelves() call is a lightweight GET request, so calling it on other Kobo settings changes (thresholds, auto-add toggle) is harmless — the shelf list simply won't change.