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>

assets/js/app.js

@@ -662,6 +662,24 @@ const EditorKeyboard = {
     }
     window.addEventListener("beforeunload", this._beforeUnload)
 
+    // Intercept LiveView navigation clicks when editor has unsaved changes.
+    // Uses capture phase to fire before LiveView's own click handler.
+    this._clickGuard = (e) => {
+      if (this.el.dataset.dirty !== "true") return
+
+      const link = e.target.closest("a[data-phx-link]")
+      if (!link) return
+
+      // Don't block clicks inside the editor itself (e.g. block controls)
+      if (this.el.contains(link)) return
+
+      if (!window.confirm("You have unsaved changes that will be lost. Leave anyway?")) {
+        e.preventDefault()
+        e.stopImmediatePropagation()
+      }
+    }
+    document.addEventListener("click", this._clickGuard, true)
+
     const prefix = this.el.dataset.eventPrefix || ""
     this._keydown = (e) => {
       if ((e.ctrlKey || e.metaKey) && e.key === "z") {
@@ -678,6 +696,7 @@ const EditorKeyboard = {
 
   destroyed() {
     window.removeEventListener("beforeunload", this._beforeUnload)
+    document.removeEventListener("click", this._clickGuard, true)
     document.removeEventListener("keydown", this._keydown)
   }
 }