add admin UX quick wins: nav guard, block descriptions, input labels

- rename "Providers" to "Print providers" in sidebar (#110)
- add LiveView navigation guard to EditorKeyboard hook — intercepts
  link clicks in capture phase when editor has unsaved changes (#103)
- add description field to all 26 block types, shown as subtitle in
  block picker; filter searches descriptions too (#104)
- add visible column headers (Label / Path) and proper sr-only labels
  with for attributes on nav editor inputs (#106)

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -91,7 +91,7 @@
               navigate={~p"/admin/providers"}
               class={admin_nav_active?(@current_path, "/admin/providers")}
             >
-              <.icon name="hero-link" class="size-5" /> Providers
+              <.icon name="hero-link" class="size-5" /> Print providers
             </.link>
           </li>
           <li>