fix(io): enforce category limit in import wizard

The import wizard now proactively checks the `MAX_CATEGORIES` limit during
the proposal and configuration steps.

Advancing is blocked with a descriptive message if the limit would be
exceeded.

Fixed UI rendering order in `ImportWizardWidget` so messages are correctly
displayed.

Add regression tests for category limit enforcement.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/ui/import_wizard_ui.rs

@@ -323,16 +323,18 @@ impl<'a> Widget for ImportWizardWidget<'a> {
                     "Enter to import, Esc to cancel",
                     Style::default().fg(Color::DarkGray),
                 );
-
-                if let Some(msg) = &self.wizard.message {
-                    let msg_y = inner.y + inner.height - 1;
-                    buf.set_string(x, msg_y, truncate(msg, w), Style::default().fg(Color::Red));
-                }
             }
             WizardStep::Done => {
                 buf.set_string(x, y, "Import complete!", Style::default().fg(Color::Green));
             }
         }
+
+        // Wizard message (errors, blocked-step explanations) takes the bottom
+        // line on every step, overriding the key hint while present.
+        if let Some(msg) = &self.wizard.message {
+            let msg_y = inner.y + inner.height - 1;
+            buf.set_string(x, msg_y, truncate(msg, w), Style::default().fg(Color::Red));
+        }
     }
 }