feat: add 't' key to transpose row/column axes

Pressing 't' swaps all Row-axis categories to Column and all
Column-axis categories to Row, leaving Page categories unchanged.
Selection and scroll offsets are reset to (0,0).

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

src/ui/app.rs

@@ -283,6 +283,11 @@ impl App {
                 }
             }
 
+            // ── Grid transpose ─────────────────────────────────────────────
+            (KeyCode::Char('t'), KeyModifiers::NONE) => {
+                self.model.active_view_mut().transpose_axes();
+            }
+
             // ── Tile movement ──────────────────────────────────────────────
             // T = enter tile select mode (single key, no Ctrl needed)
             (KeyCode::Char('T'), _) => {
@@ -1135,7 +1140,7 @@ impl App {
     /// Hint text for the status bar (context-sensitive)
     pub fn hint_text(&self) -> &'static str {
         match &self.mode {
-            AppMode::Normal => "hjkl:nav  Enter:advance  i:edit  x:clear  /:search  F/C/V:panels  T:tiles  [:]:page  ::cmd",
+            AppMode::Normal => "hjkl:nav  Enter:advance  i:edit  x:clear  t:transpose  /:search  F/C/V:panels  T:tiles  [:]:page  ::cmd",
             AppMode::Editing { .. } => "Enter:commit  Esc:cancel",
             AppMode::FormulaPanel => "n:new  d:delete  jk:nav  Esc:back",
             AppMode::FormulaEdit { .. } => "Enter:save  Esc:cancel  — type: Name = expression",