fix: localize hardcoded German strings in plugin settings views (#14)

[SeoFood]

Summary

Fixes #14 - Plugin settings views (API key test, connection status) always showed German text regardless of the app's language setting.

• Replaced all hardcoded German strings in 6 plugin settings views with IPluginLocalization calls
• Created Localization/en.json and de.json for all 6 plugins (OpenAI, Groq, Gemini, Deepgram, AssemblyAI, OpenAI Compatible)
• Fixed PluginHostServices to pass the app's configured language instead of the system culture
• Added missing CopyToAppPlugins build target to the OpenAI Compatible plugin

Test Plan

• Set app language to English, verify plugin settings show English strings
• Set app language to German, verify plugin settings show German strings
• Test API key validation flow (enter key, click Test, verify status messages)
• Test OpenAI Compatible connection flow (connect, timeout, error states)