Add support for dynamic config UI (#256)

* Add support for dynamic config UI

* Add support for section description

* Better dynamic class settings

Added dynamic_fields hash field - Stores all undeclared settings
[] method - Checks declared fields first, then falls back to dynamic hash
[]= method - Updates declared fields normally, stores others in hash
No runtime field declaration - Fields are never dynamically created on the class

* FIX proper lookup for provider keys

- Also validate configurable values properly.
- Change Provider factory to use Rails autoloading (Zeitwerk)

* Fix factory

The derive_adapter_name method relies on string manipulation ("PlaidAccount".sub(/Account$/, "") + "Adapter" → "PlaidAdapter"), but we already have explicit registration in place.

* Make updates atomic, field-aware, and handle blanks explicitly

* Small UX detail

* Add support for PlaidEU in UI also

- This looks like partial support atm

app/views/settings/_settings_nav.html.erb

@@ -28,6 +28,7 @@ nav_sections = [
         { label: "LLM Usage", path: settings_llm_usage_path, icon: "activity" },
         { label: t(".api_keys_label"), path: settings_api_key_path, icon: "key" },
         { label: t(".self_hosting_label"), path: settings_hosting_path, icon: "database", if: self_hosted? },
+        { label: "Providers", path: settings_providers_path, icon: "plug" },
         { label: t(".imports_label"), path: imports_path, icon: "download" },
         { label: "SimpleFin", path: simplefin_items_path, icon: "building-2" }
       ]