Add Raider.IO character provider

[tdecroyere]

Adds Raider.IO as an optional character profile provider while keeping Battle.net as the default and immediate fallback.

• Introduces a small ICharacterProfileProvider boundary and adapts the existing Battle.net client to it.
• Adds an anonymous Raider.IO provider using /api/v1/characters/profile with gear and the current-season Mythic+ score.
• Reads equipped item level from Raider.IO gear data and exposes the rounded current Mythic+ score on CharacterProfile.
• Adds explicit mappings from Raider.IO class/spec/race strings to the existing WoW enums, with local fr_FR display-name mappings and English fallback for other locales.
• Adds Character:Provider, Character:Region, Character:Locale, and Character:RefreshIntervalSeconds configuration. BattleNet remains the default provider; setting Character:Provider to RaiderIO requires no credentials.
• Keeps compatibility with existing alpha configs that still store region, locale, and refresh interval under BattleNet.
• Persists the provider source, localized profile names, and Mythic+ score in the character cache.
• Shows Mythic+ · <score> in the center of the default header while idle; an active key still takes precedence and shows <dungeon> · +<level> from the local combat log.
• Adds a generic data-hidden-field overlay condition so idle state can be suppressed while live Mythic+ state exists.
• Logs Raider.IO Mythic+ scores alongside refreshed item level to make freshness easy to evaluate during alpha testing.
• Adds Raider.IO attribution in the application status/startup output and README.
• Adds regression tests for Raider.IO response mapping, item-level/score conversion, state serialization, and inverse overlay visibility.

This deliberately stays in the alpha release stage. The goal is to evaluate Raider.IO against Battle.net in real WoW/OBS use before deciding whether either provider should become the long-term default.