m4: global API-error surface (no more silent failures)

The api client now catches network failures and non-JSON bodies robustly,
and routes unexpected errors (offline / 5xx) to a global toast — 4xx stay
with the caller so forms keep their inline messages. Toast actions are now
optional (undo toasts keep their button; error toasts are message-only), and
ToastHost moved from AppShell to the app root so toasts show everywhere,
including the login screen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/stores/notes.ts

@@ -106,7 +106,8 @@ export const useNotesStore = defineStore("notes", () => {
   const setPinned = (id: string, pinned: boolean) => mutate(id, { pinned });
   const setArchived = async (id: string, archived: boolean): Promise<void> => {
     await mutate(id, { archived });
-    if (archived) useUiStore().showToast("Note archived", "Undo", () => void setArchived(id, false));
+    if (archived)
+      useUiStore().showToast("Note archived", { label: "Undo", run: () => void setArchived(id, false) });
   };
   const setColor = (id: string, color: NoteColor) => mutate(id, { color });
   const setKind = (id: string, kind: NoteKind) => mutate(id, { kind });
@@ -176,7 +177,7 @@ export const useNotesStore = defineStore("notes", () => {
 
   async function trash(id: string): Promise<void> {
     reconcile(await api.post<Note>(`/api/notes/${id}/trash`));
-    useUiStore().showToast("Note moved to trash", "Undo", () => void restore(id));
+    useUiStore().showToast("Note moved to trash", { label: "Undo", run: () => void restore(id) });
   }
 
   async function restore(id: string): Promise<void> {