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/tile_bar.rs

@@ -41,10 +41,10 @@ impl<'a> Widget for TileBar<'a> {
         for (i, cat_name) in cat_names.iter().enumerate() {
             let axis = view.axis_of(cat_name);
             let axis_symbol = match axis {
-                Axis::Row => "↕",
-                Axis::Column => "↔",
-                Axis::Page => "☰",
-                Axis::Unassigned => "─",
+                Some(Axis::Row)    => "↕",
+                Some(Axis::Column) => "↔",
+                Some(Axis::Page)   => "☰",
+                None               => "─",
             };
 
             let label = format!(" [{cat_name} {axis_symbol}] ");
@@ -54,10 +54,10 @@ impl<'a> Widget for TileBar<'a> {
                 Style::default().fg(Color::Black).bg(Color::Cyan).add_modifier(Modifier::BOLD)
             } else {
                 match axis {
-                    Axis::Row => Style::default().fg(Color::Green),
-                    Axis::Column => Style::default().fg(Color::Blue),
-                    Axis::Page => Style::default().fg(Color::Magenta),
-                    Axis::Unassigned => Style::default().fg(Color::DarkGray),
+                    Some(Axis::Row)    => Style::default().fg(Color::Green),
+                    Some(Axis::Column) => Style::default().fg(Color::Blue),
+                    Some(Axis::Page)   => Style::default().fg(Color::Magenta),
+                    None               => Style::default().fg(Color::DarkGray),
                 }
             };