Add bulk invoice import with templates and refresh import UX

Move invoice import configuration into settings, redesign the import flow with shared components and sample templates, document the demo account in README, and polish upload and button styling.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/styles/globals.css

@@ -154,7 +154,8 @@
   [data-slot="card"],
   [data-slot="dialog-content"],
   [data-slot="alert-dialog-content"],
-  [data-slot="popover-content"] {
+  [data-slot="popover-content"],
+  [data-slot="select-content"] {
     border-radius: var(--radius-lg);
   }