fix: navigation bounds and stale state after model load

- app.rs: scroll_rows (Ctrl+D/U) now clamps to the cross-product row
  count and follows the viewport, matching move_selection's behaviour.
  Previously it could push selected past the last row, causing
  selected_cell_key to return None and silently ignoring edits.
- model.rs: add normalize_view_state() which resets row/col offsets to
  zero on all views.
- main.rs, dispatch.rs, app.rs: call normalize_view_state() after every
  model replacement (initial load, :Load command, wizard import) so
  stale offsets from a previous session can't hide the grid.
- app.rs: clamp formula_cursor to the current formula list length at the
  top of handle_formula_panel_key so a model reload with fewer formulas
  can't leave the cursor pointing past the end.

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

src/ui/app.rs

@@ -594,6 +594,11 @@ impl App {
     // ── Panel key handlers ───────────────────────────────────────────────────
 
     fn handle_formula_panel_key(&mut self, key: KeyEvent) -> Result<()> {
+        // Clamp cursor in case the formula list shrank since it was last set.
+        let flen = self.model.formulas.len();
+        if flen == 0 { self.formula_cursor = 0; }
+        else { self.formula_cursor = self.formula_cursor.min(flen - 1); }
+
         match key.code {
             KeyCode::Esc | KeyCode::Tab => { self.mode = AppMode::Normal; }
             KeyCode::Char('a') | KeyCode::Char('n') | KeyCode::Char('o') => {
@@ -882,8 +887,10 @@ impl App {
                     KeyCode::Backspace => wizard.pop_name_char(),
                     KeyCode::Enter => {
                         match wizard.build_model() {
-                            Ok(model) => {
+                            Ok(mut model) => {
+                                model.normalize_view_state();
                                 self.model = model;
+                                self.formula_cursor = 0;
                                 self.dirty = true;
                                 self.status_msg = "Import successful! Press :w <path> to save.".to_string();
                                 self.mode = AppMode::Normal;
@@ -956,9 +963,16 @@ impl App {
     }
 
     fn scroll_rows(&mut self, delta: i32) {
+        let row_max = {
+            let view = match self.model.active_view() { Some(v) => v, None => return };
+            let row_cats: Vec<String> = view.categories_on(Axis::Row).into_iter().map(String::from).collect();
+            cross_product_strs(&row_cats, &self.model, view).len().saturating_sub(1)
+        };
         if let Some(view) = self.model.active_view_mut() {
-            let new_r = (view.selected.0 as i32 + delta).max(0) as usize;
-            view.selected.0 = new_r;
+            let nr = (view.selected.0 as i32 + delta).clamp(0, row_max as i32) as usize;
+            view.selected.0 = nr;
+            if nr < view.row_offset { view.row_offset = nr; }
+            if nr >= view.row_offset + 20 { view.row_offset = nr.saturating_sub(19); }
         }
     }