refactor(design-system): extend DS::Alert and migrate 9 inline alert blocks (#1731)

* feat(design-system): add info semantic color token

Mirrors success/warning/destructive: --color-info maps to blue-600 in
light mode, blue-500 in dark mode. Unblocks the DS::Alert info variant
from carrying a raw 'blue-600' literal in icon_color and lets surface
tokens use bg-info/N alpha modifiers like the rest of the system.

Refs #1715

* refactor(design-system): adopt semantic tokens and add body slot in DS::Alert

Replaces the bg-{blue,green,yellow,red}-50 / text-{...}-700 / border-{...}-200
palette block in DS::Alert with semantic alpha-modifier surfaces
(bg-{info,success,warning,destructive}/10 + matching /20 borders).
Drops the 'blue-600' literal that icon_color was returning for the
info variant; helpers#icon now accepts color: :info backed by the
new --color-info token.

Adds an optional title: kwarg and an opt-in block-content slot so
rich alerts (title + paragraph, lists, embedded actions) can render
without callers reaching for a hand-rolled flex layout. The existing
message: API stays backward-compatible — nothing in the codebase that
already calls DS::Alert.new(message: ..., variant: ...) needs to change.

Lookbook gains with_title and with_body_slot examples covering the
new shapes.

Refs #1715

* refactor(views): migrate api_keys, hostings, lunchflow alerts to DS::Alert

Cleans up nine bespoke alert blocks that hand-rolled the same
flex + icon + bordered-surface shape DS::Alert already provides:

- settings/api_keys/{new,created,created.turbo_stream}.html.erb — three
  near-identical 'Security Warning' / 'Important Security Note' boxes
  using the broken bg-warning-50 / text-warning-700 raw-palette pair.
- settings/hostings/{_alpha_vantage,_eodhd,_yahoo_finance,_twelve_data,_provider_selection}_settings.html.erb —
  five amber-50 / amber-200 warning boxes covering rate-limit notes,
  health-check failure messaging, and the env-configured override
  banner. The twelve_data plan-restriction block keeps its bullet
  list and pricing link inside the new DS::Alert body slot.
- lunchflow_items/{_api_error,_setup_required}.html.erb — two modal
  alert headers whose flex+icon scaffolding now collapses onto
  DS::Alert. The surrounding bg-surface 'Common issues' / 'Setup
  steps' info cards stay as-is; this PR only touches the alert
  shape itself.

No functional or behavioural changes. Locale keys preserved.
amber-* palette uses on the alerts disappear; remaining bg-amber-*
hits in the codebase live outside the alert pattern and stay for
follow-up sub-PRs of #1715.

Refs #1715

test/components/previews/alert_component_preview.rb

@@ -1,7 +1,34 @@
 class AlertComponentPreview < Lookbook::Preview
   # @param message text
+  # @param title text
   # @param variant select [info, success, warning, error]
-  def default(message: "This is an alert message.", variant: :info)
-    render DS::Alert.new(message: message, variant: variant.to_sym)
+  def default(message: "This is an alert message.", title: nil, variant: :info)
+    render DS::Alert.new(message: message, title: title.presence, variant: variant.to_sym)
+  end
+
+  # @param variant select [info, success, warning, error]
+  def with_title(variant: :warning)
+    render DS::Alert.new(
+      message: "Heads up — this account hasn't synced in 7 days.",
+      title: "Stale connection",
+      variant: variant.to_sym
+    )
+  end
+
+  # @param variant select [info, success, warning, error]
+  def with_body_slot(variant: :error)
+    render DS::Alert.new(title: "We couldn't process this request", variant: variant.to_sym) do
+      tag.div do
+        safe_join([
+          tag.p("Verify the values you submitted and try again. If the issue persists, contact support.", class: "text-secondary"),
+          tag.ul(class: "list-disc list-inside text-secondary") do
+            safe_join([
+              tag.li("Check that all required fields are populated."),
+              tag.li("Confirm the dates fall within an open period.")
+            ])
+          end
+        ])
+      end
+    end
   end
 end