feat(filters): save current filter as smart collection — the bridge (item 3.5)

The bookshelf's save modal grows a second action: "Save as Collection"
POSTs the current (completed, scope-stamped) tree to
/api/collections — the same validated write path every tree
collection uses — and lands you on the new collection's page. The qf
engine then drives it everywhere: the shelf's live shelf-count, the
collections page, the replay chain for episode trees. The legacy
"Save Filter" (bookmark the wire params) stays as the sibling action.

This is the plan's bridge: a book search graduated into a live shelf
with one click, authored in the ONE filter system.

web/src/bookshelf.ts

@@ -179,6 +179,45 @@ Alpine.data("bookshelf", () => ({
     // Show success toast
     showToast("Filter saved successfully", "success");
   },
+  // Item 3.5 bridge: the current tree becomes a live smart collection.
+  // The qf engine then drives it everywhere — shelf, collections page,
+  // play chains — with zero re-authoring.
+  async saveFilterAsCollection() {
+    const token = localStorage.getItem("token");
+    if (!token) {
+      showToast("Not authenticated", "error");
+      return;
+    }
+    const nameInput = document.getElementById("save-filter-name") as HTMLInputElement | null;
+    const name = nameInput?.value?.trim() ?? "";
+    if (!name) {
+      showToast("Give the collection a name first", "error");
+      return;
+    }
+    const builder = builderData();
+    const tree = builder?.scopedTree?.();
+    if (!tree) {
+      showToast("Add a complete filter rule first", "error");
+      return;
+    }
+    try {
+      const response = await fetch("/api/collections", {
+        method: "POST",
+        headers: { "Content-Type": "application/json", Authorization: `Bearer ${token}` },
+        body: JSON.stringify({ name, filter_tree: tree }),
+      });
+      if (response.ok) {
+        this.showSaveModal = false;
+        showToast("Saved as a smart collection", "success");
+        window.location.href = `/collections/${((await response.json()) as { id: string }).id}`;
+      } else {
+        const body = (await response.json().catch(() => ({}))) as { error?: string };
+        showToast(body.error || "Failed to save the collection", "error");
+      }
+    } catch {
+      showToast("Failed to save the collection", "error");
+    }
+  },
   // Load a saved filter into the form
   async loadFilter(event: Event) {
     const button = event.target as HTMLElement;