fix(canvas): add focus-visible to secrets-tab add/clear-search buttons

Both .secrets-tab__add-btn and .secrets-tab__clear-search were missing
:focus-visible rules, violating WCAG 2.4.7 (Focus Visible). Keyboard
users tabbing to these buttons would see no visible focus indicator.

Fix: add :focus-visible { outline: var(--focus-ring); outline-offset:
var(--focus-ring-offset); } to both selectors, consistent with the
existing focus-visible pattern used throughout the settings panel.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/styles/settings-panel.css

@@ -411,6 +411,12 @@
   color: #f4f4f5;
 }
 
+/* WCAG 2.4.7 — keyboard focus indicator */
+.secrets-tab__add-btn:focus-visible {
+  outline: var(--focus-ring);
+  outline-offset: var(--focus-ring-offset);
+}
+
 /* ── Shared UI ─────────────────────────────────────── */
 
 .key-value-field {
@@ -601,6 +607,12 @@
   font-size: 14px;
 }
 
+/* WCAG 2.4.7 — keyboard focus indicator */
+.secrets-tab__clear-search:focus-visible {
+  outline: var(--focus-ring);
+  outline-offset: var(--focus-ring-offset);
+}
+
 /* ── Delete confirmation dialog ────────────────────── */
 
 .delete-dialog__overlay {