feat(ux): Sticky Save/Cancel buttons on forms (#38)

- Add sticky positioning to form action buttons (always visible at bottom)
- Add Ctrl+S / Cmd+S keyboard shortcut to save forms
- Create reusable useFormShortcuts hook
- Apply to all create forms: spool, filament, vendor

Fixes: https://github.com/Donkie/Spoolman/issues/274

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/src/utils/useFormShortcuts.ts

@@ -0,0 +1,26 @@
+import { useEffect } from "react";
+
+/**
+ * Hook to add keyboard shortcuts to forms.
+ * - Ctrl+S / Cmd+S: Save (calls onSave callback)
+ * - Escape: Cancel (calls onCancel callback if provided)
+ */
+export function useFormShortcuts(onSave: () => void, onCancel?: () => void) {
+  useEffect(() => {
+    const handleKeyDown = (e: KeyboardEvent) => {
+      // Ctrl+S or Cmd+S to save
+      if ((e.ctrlKey || e.metaKey) && e.key === "s") {
+        e.preventDefault();
+        onSave();
+      }
+      // Escape to cancel (if handler provided)
+      if (e.key === "Escape" && onCancel) {
+        e.preventDefault();
+        onCancel();
+      }
+    };
+
+    document.addEventListener("keydown", handleKeyDown);
+    return () => document.removeEventListener("keydown", handleKeyDown);
+  }, [onSave, onCancel]);
+}