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>

assets/css/admin/components.css

@@ -553,6 +553,7 @@
 
 .admin-input-error {
   border-color: var(--t-status-error);
+  border-width: 2px;
 
   &:focus {
     border-color: var(--t-status-error);
@@ -876,13 +877,20 @@
 .admin-error {
   display: flex;
   align-items: center;
-  gap: 0.5rem;
+  gap: 0.375rem;
   margin-top: 0.375rem;
   font-size: 0.875rem;
   line-height: 1.25rem;
+  font-weight: 600;
   color: var(--t-status-error);
 }
 
+.admin-error-icon {
+  width: 1em;
+  height: 1em;
+  flex-shrink: 0;
+}
+
 /* ── Link ── */
 
 .admin-link {
@@ -4774,7 +4782,7 @@
 .truncate { overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
 .ml-1 { margin-inline-start: 0.25rem; }
 
-.external-link-icon {
+.external-link-icon.external-link-icon {
   width: 0.75em;
   height: 0.75em;
   margin-inline-start: 0.25em;