feat(ui): global confirm modal — browser dialogs retired sitewide (issues #5/#4/#7)

ONE styled modal answers every hx-confirm in the app: a body-level
htmx:confirm interceptor (web/src/confirm.ts, canonical htmx pattern)
preventDefaults and shows the dialog, issuing the request on Confirm.
Nine hx-confirm call sites upgrade without touching them. The modal
lives in the header partial so every page has it; confirmDialog() is
exported for JS call sites and degrades to the native dialog if the
header is absent (bare layouts).

Migrated the five window.confirm sites (admin.ts ×3, book-detail.ts ×2)
to the same promise API.

Podcast feed detail (issues #4/#7): the unsubscribe copy is
role-aware — non-admins get 'Unsubscribe from this feed?' with silent
retention (file state is admin knowledge); an admin who is the LAST
subscriber gets a three-choice modal instead: Unsubscribe (files kept,
the D6 default — sliding-window feeds make purge unrecoverable) /
Remove feed + files / Cancel. Admins with co-subscribers keep the
files-note confirm.

web/src/admin.ts

@@ -1,3 +1,4 @@
+import { confirmDialog } from "./confirm";
 import { Alpine } from "./alpine";
 import { showToast } from "./toast";
 
@@ -423,9 +424,9 @@ async function purgeArchivedItems(): Promise<void> {
   if (!token) return;
 
   if (
-    !window.confirm(
+    !(await confirmDialog(
       "Permanently delete all archived items? Their reading progress, notes, and highlights will be lost.",
-    )
+    ))
   ) {
     return;
   }
@@ -475,9 +476,9 @@ async function deleteArchivedItem(id: string): Promise<void> {
   const token = requireToken();
   if (!token) return;
   if (
-    !window.confirm(
+    !(await confirmDialog(
       "Permanently delete this item? Its reading progress, notes, and highlights will be lost.",
-    )
+    ))
   ) {
     return;
   }
@@ -504,9 +505,9 @@ async function mergeArchivedItem(sourceId: string, targetId: string): Promise<vo
   const token = requireToken();
   if (!token) return;
   if (
-    !window.confirm(
+    !(await confirmDialog(
       "Merge this archived copy's reading progress, notes, and highlights into the active copy? The archived entry is removed.",
-    )
+    ))
   ) {
     return;
   }