fix: formula correctness and command validation bugs

- model.rs: division by zero now returns Empty instead of 0 so the cell
  visually signals the error rather than silently showing a wrong value.
  Updated the test to assert Empty.
- parser.rs: missing closing ')' in aggregate functions (SUM, AVG, etc.)
  and IF(...) now returns a parse error instead of silently succeeding,
  preventing malformed formulas from evaluating with unexpected results.
- dispatch.rs: SetCell validates that every category in the coords
  exists before mutating anything; previously a typo in a category name
  silently wrote an orphaned cell (stored but never visible in any grid)
  and returned ok. Now returns an error immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/model/model.rs

@@ -123,6 +123,16 @@ impl Model {
         Ok(())
     }
 
+    /// Reset all view scroll offsets to zero.
+    /// Call this after loading or replacing a model so stale offsets don't
+    /// cause the grid to render an empty area.
+    pub fn normalize_view_state(&mut self) {
+        for view in self.views.values_mut() {
+            view.row_offset = 0;
+            view.col_offset = 0;
+        }
+    }
+
     /// Return all category names
     pub fn category_names(&self) -> Vec<&str> {
         self.categories.keys().map(|s| s.as_str()).collect()
@@ -182,7 +192,7 @@ impl Model {
                         "+" => lv + rv,
                         "-" => lv - rv,
                         "*" => lv * rv,
-                        "/" => if rv == 0.0 { 0.0 } else { lv / rv },
+                        "/" => { if rv == 0.0 { return None; } lv / rv }
                         "^" => lv.powf(rv),
                         _ => return None,
                     })
@@ -482,7 +492,7 @@ mod formula_tests {
     }
 
     #[test]
-    fn division_by_zero_yields_zero() {
+    fn division_by_zero_yields_empty() {
         let mut m = Model::new("Test");
         m.add_category("Measure").unwrap();
         m.add_category("Region").unwrap();
@@ -494,7 +504,8 @@ mod formula_tests {
         m.set_cell(coord(&[("Measure", "Revenue"), ("Region", "East")]), CellValue::Number(100.0));
         m.set_cell(coord(&[("Measure", "Zero"),    ("Region", "East")]), CellValue::Number(0.0));
         m.add_formula(parse_formula("Result = Revenue / Zero", "Measure").unwrap());
-        assert_eq!(m.evaluate(&coord(&[("Measure", "Result"), ("Region", "East")])), CellValue::Number(0.0));
+        // Division by zero must yield Empty, not 0, so the user sees a blank not a misleading zero.
+        assert_eq!(m.evaluate(&coord(&[("Measure", "Result"), ("Region", "East")])), CellValue::Empty);
     }
 
     #[test]