add forgiving API key validation with inline errors

Add KeyValidation module for format-checking API keys before
attempting connections. Auto-strips whitespace, detects common
mistakes (e.g. pasting a Stripe publishable key), and returns
helpful error messages.

Inline field errors across all three entry points:
- Setup wizard: provider + Stripe keys
- Admin provider form: simplified to single Connect button
- Email settings: per-field errors instead of flash toasts

Also: plain text inputs for all API keys (not password fields),
accessible error states (aria-invalid, role=alert, thick border,
bold text), inner_block slot declaration on error component.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/live/admin/providers/form.html.heex

@@ -32,7 +32,7 @@
 
     <.input
       field={@form[:api_key]}
-      type="password"
+      type="text"
       label={"#{@provider.name} API key"}
       placeholder={
         if @live_action == :edit,
@@ -42,44 +42,14 @@
       autocomplete="off"
     />
 
-    <div class="admin-inline-group">
-      <button
-        type="button"
-        class="admin-btn admin-btn-outline admin-btn-sm"
-        phx-click="test_connection"
-        disabled={@testing}
-      >
-        <.icon
-          name={if @testing, do: "hero-arrow-path", else: "hero-signal"}
-          class={if @testing, do: "size-4 animate-spin", else: "size-4"}
-        />
-        {if @testing, do: "Checking...", else: "Check connection"}
-      </button>
-
-      <%= if @test_result do %>
-        <%= case @test_result do %>
-          <% {:ok, _info} -> %>
-            <span class="admin-status-success">
-              <.icon name="hero-check-circle" class="size-4" />
-              Connected to {connection_name(@test_result) || @provider.name}
-            </span>
-          <% {:error, reason} -> %>
-            <span class="admin-status-error">
-              <.icon name="hero-x-circle" class="size-4" />
-              {format_error(reason)}
-            </span>
-        <% end %>
-      <% end %>
-    </div>
-
     <%= if @live_action == :edit do %>
       <.input field={@form[:enabled]} type="checkbox" label="Connection enabled" />
     <% end %>
 
     <div class="admin-form-actions">
-      <.button type="submit" disabled={@testing}>
+      <.button type="submit">
         {if @live_action == :new,
-          do: "Connect to #{@provider.name}",
+          do: "Connect",
           else: "Save changes"}
       </.button>
       <.link navigate={~p"/admin/providers"} class="admin-btn admin-btn-ghost">