sl0p: configurable pane title alignment

Adds `ui { pane_frames { title_alignment "left"|"center"|"right" } }`.

Full-frame panes previously hardcoded the title flush left, while frameless
panes (pane_frame_style "titles") already centered it via
compose_bracketed_title. This brings full frames up to parity and makes the
position configurable.

Implementation notes:
- The value rides on `Style`, which is already threaded to FrameParams, so no
  new plumbing through Tab/PaneContentsAndUi was needed.
- It is deliberately NOT round-tripped over the plugin protobuf boundary:
  plugins have no use for frame title alignment and adding a proto field
  would force regenerating the committed prost assets in assets/prost/.
- Left keeps the original code path untouched, so default configs render
  exactly as before.
- The geometry loop clamps the title start against the right-side indicators
  and breaks on `>=` rather than `==`, so a mis-clamped segment cannot spin.

Tests: alignment ordering, frame width preservation across widths, and
narrow-pane safety (cols 1..24 must not panic or corrupt width).

1 files changed, 25 insertions, 0 deletions

diff --git a/zellij-utils/src/kdl/mod.rs b/zellij-utils/src/kdl/mod.rs
index 8e86445f..bc188aed 100644
--- a/zellij-utils/src/kdl/mod.rs
+++ b/zellij-utils/src/kdl/mod.rs
@@ -5524,9 +5524,21 @@ impl UiConfig {
                     .unwrap_or(false);
             let hide_session_name =
                 kdl_get_child_entry_bool_value!(pane_frames, "hide_session_name").unwrap_or(false);
+            let title_alignment = match kdl_children_property_first_arg_as_string!(
+                pane_frames,
+                "title_alignment"
+            ) {
+                Some(string) => {
+                    crate::data::PaneTitleAlignment::from_str(string).map_err(|e| {
+                        ConfigError::new_kdl_error(e, kdl_ui_config.span().offset(), kdl_ui_config.span().len())
+                    })?
+                },
+                None => crate::data::PaneTitleAlignment::default(),
+            };
             let frame_config = FrameConfig {
                 rounded_corners,
                 hide_session_name,
+                title_alignment,
             };
             ui_config.pane_frames = frame_config;
         }
@@ -5550,6 +5562,19 @@ impl UiConfig {
             hide_session_name.push(KdlValue::Bool(true));
             frame_config_children.nodes_mut().push(hide_session_name);
         }
+        if self.pane_frames.title_alignment != crate::data::PaneTitleAlignment::default() {
+            has_ui_config = true;
+            let mut title_alignment = KdlNode::new("title_alignment");
+            title_alignment.push(KdlValue::String(
+                match self.pane_frames.title_alignment {
+                    crate::data::PaneTitleAlignment::Left => "left",
+                    crate::data::PaneTitleAlignment::Center => "center",
+                    crate::data::PaneTitleAlignment::Right => "right",
+                }
+                .to_string(),
+            ));
+            frame_config_children.nodes_mut().push(title_alignment);
+        }
         if has_ui_config {
             frame_config.set_children(frame_config_children);
             ui_config_children.nodes_mut().push(frame_config);