refactor(ui): improve rendering, feedback, and help system

Improve UI rendering and feedback:

- `src/draw.rs` :
    - Automatically enter Help mode if the model is empty.
    - Render the `HelpWidget` with the current help page.
    - Render the `WhichKeyWidget` when a transient keymap is active.
- `src/ui/tile_bar.rs` : Use more descriptive labels for axes (e.g., "Row",
  "Col", "Pag").
- `src/ui/view_panel.rs` :
    - Include an axis summary (e.g., "R:Region C:Product") next to view
      names.
    - Refactor `ViewContent` to hold a reference to the `Model` .
- `src/ui/effect.rs` : Add error feedback to `AddItem` , `AddItemInGroup` ,
  and `AddFormula` when operations fail.
- `src/ui/help.rs` : Refactor `HelpWidget` into a multi-page system.

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

src/ui/effect.rs

@@ -35,6 +35,8 @@ impl Effect for AddItem {
     fn apply(&self, app: &mut App) {
         if let Some(cat) = app.model.category_mut(&self.category) {
             cat.add_item(&self.item);
+        } else {
+            app.status_msg = format!("Unknown category '{}'", self.category);
         }
     }
 }
@@ -49,6 +51,8 @@ impl Effect for AddItemInGroup {
     fn apply(&self, app: &mut App) {
         if let Some(cat) = app.model.category_mut(&self.category) {
             cat.add_item_in_group(&self.item, &self.group);
+        } else {
+            app.status_msg = format!("Unknown category '{}'", self.category);
         }
     }
 }
@@ -76,8 +80,13 @@ pub struct AddFormula {
 }
 impl Effect for AddFormula {
     fn apply(&self, app: &mut App) {
-        if let Ok(formula) = crate::formula::parse_formula(&self.raw, &self.target_category) {
-            app.model.add_formula(formula);
+        match crate::formula::parse_formula(&self.raw, &self.target_category) {
+            Ok(formula) => {
+                app.model.add_formula(formula);
+            }
+            Err(e) => {
+                app.status_msg = format!("Formula error: {e}");
+            }
         }
     }
 }