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/tile_bar.rs

@@ -26,10 +26,10 @@ impl<'a> TileBar<'a> {
     }
     fn axis_display(axis: Axis) -> (&'static str, Color) {
         match axis {
-            Axis::Row => ("|", Color::Green),
-            Axis::Column => ("-", Color::Blue),
-            Axis::Page => ("=", Color::Magenta),
-            Axis::None => (".", Color::DarkGray),
+            Axis::Row => ("Row", Color::Green),
+            Axis::Column => ("Col", Color::Blue),
+            Axis::Page => ("Pag", Color::Magenta),
+            Axis::None => ("·", Color::DarkGray),
         }
     }
 }