Toast action results and confirm destructive actions in one dialog

Inline banners and per-row status text reported action results all over
the app; they are replaced by a small toast stack (bottom-right, Level 3
floating well styling) that only speaks for actions: successes fade,
errors stay until dismissed, and form-field validation stays inline.

Destructive actions no longer use bespoke inline confirm steps (the
library detail's Confirm delete button) or fire immediately (duplicate
copies/items, delete page selections, temp cleanup, upload discard, job
cancellation): they all go through one promise-based confirm dialog
(confirmAction) with a danger variant, Escape/backdrop to cancel.

frontend/src/components/AppShell.tsx

@@ -4,8 +4,10 @@ import { useEffect, useState } from "react";
 import { Link, NavLink, Outlet, useLocation, useNavigationType } from "react-router-dom";
 
 import { CommandPalette } from "@/components/CommandPalette";
+import { ConfirmDialog } from "@/components/ConfirmDialog";
 import { StatusFooter } from "@/components/StatusFooter";
 import { StatusPill } from "@/components/StatusPill";
+import { Toasts } from "@/components/Toasts";
 import { api } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import type { SystemStatus } from "@/lib/types";
@@ -182,6 +184,8 @@ export function AppShell() {
         )}
       />
       <CommandPalette open={paletteOpen} onClose={() => setPaletteOpen(false)} />
+      <ConfirmDialog />
+      <Toasts />
     </div>
   );
 }