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/overrides.css

@@ -6,3 +6,15 @@
 .ant-form {
   max-width: 800px;
 }
+
+/* Sticky Save/Cancel buttons on forms (#38) */
+.ant-card-actions {
+  position: sticky;
+  bottom: 0;
+  z-index: 100;
+  background: var(--ant-color-bg-container, #fff);
+  border-top: 1px solid var(--ant-color-border-secondary, #f0f0f0);
+  margin: 0 -24px -24px -24px;
+  padding: 12px 24px;
+  border-radius: 0 0 8px 8px;
+}