Make default-board.json content translatable

Describe the issue

When a new user opens Deck for the first time, the default welcome board is created.

Most of the Deck interface is correctly displayed in French when the user language is set to French, but some content inside the default welcome board remains in English.

For example, the card description contains:

## 🚦 Manage your boards and cards

Drag your cards around the board to change position in the list or move them to other lists.

Archive cards to remove them from the board without losing your content.

After checking the source files, this content appears to come from:

lib/Service/fixtures/default-board.json

This content does not seem to be exposed through the usual l10n translation mechanism.

The default welcome board may be fully localized according to the user language.

For example, when the user language is French, the default board content should be created in French, including the card descriptions.

Some card titles or interface elements are translated, but the long card description remains in English.

Proposed solution :
Make the default board fixture content translatable, either by:

1. moving the fixture strings to the l10n system;
2. generating the default board content through translated strings;
3. or supporting localized fixture files such as default-board.fr.json.

Environment
Nextcloud version: 33.0.x

Deck app version: 1.17.1

User language: French

Server default language: French

File involved: lib/Service/fixtures/default-board.json

Thank you for your work and for your investment.

Start by reading lib/Service/fixtures/default-board.json and tracing how the default welcome board is created and how existing l10n strings are loaded. Make the fixture's card descriptions follow the user's language, then verify that French users receive localized default-board content rather than the current English text.