Improve convert-to-trade security selection with search-first UX (#703)

* Enhance security handling logic:
- Prioritize user's country in sorting securities and country codes.
- Add comprehensive mapping for MIC codes to user-friendly exchange names.
- Revamp combobox to consistently pull from a provider when available.
- Improve handling of custom ticker and exchange input fields.

* Localize securities combobox display and exchange labels.

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Co-authored-by: luckyPipewrench <luckypipewrench@proton.me>

app/controllers/securities_controller.rb

@@ -2,7 +2,7 @@ class SecuritiesController < ApplicationController
   def index
     @securities = Security.search_provider(
       params[:q],
-      country_code: params[:country_code] == "US" ? "US" : nil
+      country_code: params[:country_code].presence
     )
   end
 end