Add invoice drafts, read-only issued state and keyboard shortcuts

Views stay mounted so switching tabs no longer loses a half-typed invoice.
The form autosaves a draft after 1 s, drafts can be resumed or deleted,
and after issuing the form is read-only with Export again, Duplicate and
New invoice actions. A failed export keeps the issued invoice and offers a
retry instead of issuing again. Adds Ctrl+N/S/Enter/L and a shortcut sheet;
F5 and Ctrl+R are blocked in production builds.

src/hooks/useShortcuts.ts

@@ -0,0 +1,62 @@
+import { useEffect, useRef } from "react";
+
+export interface ShortcutHandlers {
+  onNew?: () => void;
+  onSaveDraft?: () => void;
+  onIssue?: () => void;
+  onAddLine?: () => void;
+  onHelp?: () => void;
+}
+
+const isTyping = (target: EventTarget | null) => {
+  const el = target as HTMLElement | null;
+  if (!el) return false;
+  return el.tagName === "INPUT" || el.tagName === "TEXTAREA" || el.tagName === "SELECT" || el.isContentEditable;
+};
+
+/**
+ * Global keyboard shortcuts. Ctrl combinations work everywhere; the bare `?` is ignored
+ * while typing. F5 / Ctrl+R are blocked in production builds because a webview reload
+ * would discard in-memory state.
+ */
+export function useShortcuts(handlers: ShortcutHandlers) {
+  const ref = useRef(handlers);
+  ref.current = handlers;
+
+  useEffect(() => {
+    const onKey = (event: KeyboardEvent) => {
+      const ctrl = event.ctrlKey || event.metaKey;
+      const key = event.key.toLowerCase();
+
+      if (import.meta.env.PROD && (event.key === "F5" || (ctrl && key === "r"))) {
+        event.preventDefault();
+        return;
+      }
+
+      const run = (fn?: () => void) => {
+        event.preventDefault();
+        fn?.();
+      };
+
+      if (ctrl && !event.shiftKey && !event.altKey) {
+        if (key === "n") return run(ref.current.onNew);
+        if (key === "s") return run(ref.current.onSaveDraft);
+        if (key === "l") return run(ref.current.onAddLine);
+        if (key === "enter") return run(ref.current.onIssue);
+      }
+      if (!ctrl && !event.altKey && event.key === "?" && !isTyping(event.target)) {
+        run(ref.current.onHelp);
+      }
+    };
+    window.addEventListener("keydown", onKey);
+    return () => window.removeEventListener("keydown", onKey);
+  }, []);
+}
+
+export const SHORTCUT_LIST: { keys: string; action: string }[] = [
+  { keys: "Ctrl + N", action: "New invoice" },
+  { keys: "Ctrl + S", action: "Save draft" },
+  { keys: "Ctrl + Enter", action: "Issue and export PDF" },
+  { keys: "Ctrl + L", action: "Add line item" },
+  { keys: "?", action: "Show this list" },
+];