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

app/views/lunchflow_items/_api_error.html.erb

@@ -4,13 +4,11 @@
     <% dialog.with_header(title: "Lunch Flow Connection Error") %>
     <% dialog.with_body do %>
       <div class="space-y-4">
-        <div class="flex items-start gap-3">
-          <%= icon("alert-circle", class: "text-destructive w-5 h-5 shrink-0 mt-0.5") %>
-          <div class="text-sm">
-            <p class="font-medium text-primary mb-2">Unable to connect to Lunch Flow</p>
-            <p class="text-secondary"><%= error_message %></p>
-          </div>
-        </div>
+        <%= render DS::Alert.new(
+          title: "Unable to connect to Lunch Flow",
+          message: error_message,
+          variant: :error
+        ) %>
 
         <div class="bg-surface rounded-lg p-4 space-y-2 text-sm">
           <p class="font-medium text-primary">Common Issues:</p>