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/features/upload/UploadPage.tsx

@@ -34,7 +34,9 @@ import type {
   TempUpload,
 } from "@/lib/types";
 import { useAuth } from "@/store/auth";
+import { confirmAction } from "@/store/confirm";
 import { useE621 } from "@/store/e621";
+import { toast } from "@/store/toasts";
 
 type UploadStatus = "queued" | "uploading" | "done" | "error";
 
@@ -241,7 +243,7 @@ function MetadataModal({
       void queryClient.invalidateQueries({ queryKey: ["files"] });
       onClose();
     } catch (err) {
-      setError(describeError(err));
+      toast.error(describeError(err));
       setBusy(false);
     }
   }
@@ -284,11 +286,22 @@ function MetadataModal({
     });
   }
 
-  function discardUpload() {
-    void api(`/api/uploads/${temp.temp_id}/`, { method: "DELETE" }).then(() => {
-      void queryClient.invalidateQueries({ queryKey: ["uploads"] });
-      onClose();
+  async function discardUpload() {
+    const confirmed = await confirmAction({
+      title: "Discard this upload?",
+      description: `${temp.original_filename} is deleted from the staging folder; it never entered the library.`,
+      confirmLabel: "Discard",
+      danger: true,
     });
+    if (!confirmed) return;
+    try {
+      await api(`/api/uploads/${temp.temp_id}/`, { method: "DELETE" });
+      void queryClient.invalidateQueries({ queryKey: ["uploads"] });
+      toast.ok("Upload discarded.");
+      onClose();
+    } catch (error) {
+      toast.error(errorMessage(error));
+    }
   }
 
   return (
@@ -416,7 +429,7 @@ function MetadataModal({
                     </p>
                     <button
                       type="button"
-                      onClick={discardUpload}
+                      onClick={() => void discardUpload()}
                       className="mt-2 rounded-md px-1.5 py-0.5 font-mono text-[10px] text-ctp-red transition hover:bg-ctp-red/15"
                     >
                       discard this upload
@@ -591,9 +604,7 @@ function MetadataModal({
             )}
 
             {error ? (
-              <p className="rounded-md border border-ctp-red/30 bg-ctp-red/10 px-3 py-2 text-sm text-ctp-red">
-                {error}
-              </p>
+              <p className="text-xs text-ctp-red">{error}</p>
             ) : null}
             {busy ? (
               <p className="flex items-center gap-2 text-xs text-ctp-overlay0">