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",

src/view/view.rs

@@ -112,6 +112,18 @@ impl View {
         self.hidden_items.get(cat_name).map(|s| s.contains(item_name)).unwrap_or(false)
     }
 
+    /// Swap all Row categories to Column and all Column categories to Row.
+    /// Page categories are unaffected.
+    pub fn transpose_axes(&mut self) {
+        let rows: Vec<String> = self.categories_on(Axis::Row).iter().map(|s| s.to_string()).collect();
+        let cols: Vec<String> = self.categories_on(Axis::Column).iter().map(|s| s.to_string()).collect();
+        for cat in &rows { self.set_axis(cat, Axis::Column); }
+        for cat in &cols { self.set_axis(cat, Axis::Row); }
+        self.selected = (0, 0);
+        self.row_offset = 0;
+        self.col_offset = 0;
+    }
+
     /// Cycle axis for a category: Row → Column → Page → Row
     pub fn cycle_axis(&mut self, cat_name: &str) {
         let next = match self.axis_of(cat_name) {
@@ -171,6 +183,34 @@ mod tests {
         assert_eq!(v.categories_on(Axis::Page),   vec!["Time"]);
     }
 
+    #[test]
+    fn transpose_axes_swaps_row_and_column() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        // Default: Region=Row, Product=Column
+        assert_eq!(v.axis_of("Region"),  Axis::Row);
+        assert_eq!(v.axis_of("Product"), Axis::Column);
+        v.transpose_axes();
+        assert_eq!(v.axis_of("Region"),  Axis::Column);
+        assert_eq!(v.axis_of("Product"), Axis::Row);
+    }
+
+    #[test]
+    fn transpose_axes_leaves_page_categories_unchanged() {
+        let mut v = view_with_cats(&["Region", "Product", "Time"]);
+        // Default: Region=Row, Product=Column, Time=Page
+        v.transpose_axes();
+        assert_eq!(v.axis_of("Time"), Axis::Page);
+    }
+
+    #[test]
+    fn transpose_axes_is_its_own_inverse() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.transpose_axes();
+        v.transpose_axes();
+        assert_eq!(v.axis_of("Region"),  Axis::Row);
+        assert_eq!(v.axis_of("Product"), Axis::Column);
+    }
+
     #[test]
     #[should_panic(expected = "axis_of called for category not registered")]
     fn axis_of_unknown_category_panics() {