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).

2 files changed, 6 insertions, 0 deletions

diff --git a/zellij-utils/src/plugin_api/event.rs b/zellij-utils/src/plugin_api/event.rs
index 970f5e2c..0189d11b 100644
--- a/zellij-utils/src/plugin_api/event.rs
+++ b/zellij-utils/src/plugin_api/event.rs
@@ -2492,6 +2492,7 @@ fn serialize_mode_update_event_with_non_default_values() {
             // TODO: replace default
             rounded_corners: true,
             hide_session_name: false,
+            pane_title_alignment: Default::default(),
         },
         capabilities: PluginCapabilities { arrow_fonts: false },
         session_name: Some("my awesome test session".to_owned()),
diff --git a/zellij-utils/src/plugin_api/style.rs b/zellij-utils/src/plugin_api/style.rs
index eb9eebcf..2f3ff298 100644
--- a/zellij-utils/src/plugin_api/style.rs
+++ b/zellij-utils/src/plugin_api/style.rs
@@ -21,6 +21,11 @@ impl TryFrom<ProtobufStyle> for Style {
             colors: s,
             rounded_corners: protobuf_style.rounded_corners,
             hide_session_name: protobuf_style.hide_session_name,
+            // deliberately not carried over the plugin protobuf boundary; see
+            // the field docs on Style. Plugins never need the frame's title
+            // alignment, and adding a proto field would mean regenerating the
+            // committed prost assets.
+            pane_title_alignment: Default::default(),
         })
     }
 }