fix(ai): valid Tailwind token + base_url URL validation

- Data-handling block in _llm_provider_selector swaps the invalid
  bg-surface-secondary token for bg-container-inset, matching the
  inset-card pattern used elsewhere in sure-design-system/components.css.
  bg-surface-secondary is not defined anywhere in the design system —
  Tailwind treated it as a no-op, so the block rendered with no
  background contrast.
- Settings::HostingsController validates anthropic_base_url as a
  URI::HTTP (catches https too) and raises Setting::ValidationError
  with a localized message when the input is not parseable.
  Previously any string was persisted, surfacing as an opaque
  connection error at request time instead of an immediate UX failure.
- Blank base_url now clears the setting (was already the case but
  exercised explicitly in tests now).

app/controllers/settings/hostings_controller.rb

@@ -174,7 +174,16 @@ class Settings::HostingsController < ApplicationController
     end
 
     if hosting_params.key?(:anthropic_base_url)
-      Setting.anthropic_base_url = hosting_params[:anthropic_base_url].presence
+      raw_base_url = hosting_params[:anthropic_base_url].to_s.strip
+      if raw_base_url.blank?
+        Setting.anthropic_base_url = nil
+      else
+        parsed = URI.parse(raw_base_url) rescue nil
+        unless parsed.is_a?(URI::HTTP)
+          raise Setting::ValidationError, t(".invalid_anthropic_base_url")
+        end
+        Setting.anthropic_base_url = raw_base_url
+      end
     end
 
     if hosting_params.key?(:anthropic_model)