refactor: remove Axis::Unassigned; axis_of returns Option<Axis>

Axis::Unassigned served two purposes that Option already covers:
  1. "this category has no assignment yet" → None
  2. "this category doesn't exist" → None

By removing the variant and changing axis_of to return Option<Axis>,
callers are forced by the compiler to handle the absent-category case
explicitly (via match or unwrap_or), rather than silently treating it
like a real axis value.

SetAxis { axis: String } also upgraded to SetAxis { axis: Axis }.
Previously, constructing SetAxis with an invalid string (e.g. "diagonal")
would compile and then silently fail at dispatch. Now the type only admits
valid axis values; the dispatch string-parser is gone.

Axis gains #[serde(rename_all = "lowercase")] so existing JSON command
files (smoke.jsonl, etc.) using "row"/"column"/"page" continue to work.

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

src/ui/category_panel.rs

@@ -67,16 +67,16 @@ impl<'a> Widget for CategoryPanel<'a> {
 
             let axis = view.axis_of(cat_name);
             let axis_str = match axis {
-                Axis::Row => "Row ↕",
-                Axis::Column => "Col ↔",
-                Axis::Page => "Page ☰",
-                Axis::Unassigned => "none",
+                Some(Axis::Row)    => "Row ↕",
+                Some(Axis::Column) => "Col ↔",
+                Some(Axis::Page)   => "Page ☰",
+                None               => "none",
             };
             let axis_color = match axis {
-                Axis::Row => Color::Green,
-                Axis::Column => Color::Blue,
-                Axis::Page => Color::Magenta,
-                Axis::Unassigned => Color::DarkGray,
+                Some(Axis::Row)    => Color::Green,
+                Some(Axis::Column) => Color::Blue,
+                Some(Axis::Page)   => Color::Magenta,
+                None               => Color::DarkGray,
             };
 
             let item_count = self.model.category(cat_name).map(|c| c.items.len()).unwrap_or(0);