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.

templates/header.templ

@@ -352,4 +352,24 @@ templ SidebarSignIn(currentPath string) {
 			}
 		</div>
 	</div>
+
+	<!-- Global confirm modal (issue #5): ONE styled dialog answers every
+	     hx-confirm sitewide via the htmx:confirm interceptor in
+	     web/src/confirm.ts, and exposes the confirmDialog() promise for
+	     legacy JS call sites. Inert until shown. -->
+	<div
+		id="confirm-modal"
+		class="hidden fixed inset-0 z-[60] flex items-center justify-center p-4"
+		style="background-color: var(--surface-overlay);"
+	>
+		<div class="card w-full max-w-sm" style="box-shadow: var(--shadow-pop);">
+			<div class="p-5 border-b" style="border-color: var(--border);">
+				<p id="confirm-modal-message" class="text-sm font-medium" style="color: var(--text-primary)"></p>
+			</div>
+			<div class="p-4 flex justify-end gap-2">
+				<button id="confirm-modal-cancel" class="btn btn-secondary">Cancel</button>
+				<button id="confirm-modal-accept" class="btn btn-primary">Confirm</button>
+			</div>
+		</div>
+	</div>
 }