Unify TUI completion popup styling and preserve result identity (#46694)

## Why

Narrow completion popups can hide distinguishing filenames, parent paths, or category tags. Unified mentions also need a stable result viewport across searches and filters.

## What changed

- Use shared selection styling and `›` markers across command, file, skill, and unified mention popups.
- Give unified mentions a fixed-height picker layout with a header, filter tabs, query display, scroll indicators, and keyboard hints.
- Bound mention name columns to leave room for parent paths and measure alignment in terminal cells.
- Truncate file paths while retaining filenames and recalculating visible fuzzy highlights, preserving the full path for insertion.
- Keep category tags visible when skill and app descriptions are hidden.

## Testing

Add snapshots and assertions for narrow and clipped layouts, scrolling, empty filters, full-row selection styling, Unicode column alignment, long paths, preserved file selections, and category tags.

GitOrigin-RevId: 54c34cc2472b43bac22011206efed3041eb77308

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__apps_mentions_only_callable_installed.snap

@@ -1,13 +1,13 @@
 ---
 source: tui/src/chatwidget/tests/popups_and_settings.rs
-assertion_line: 2317
 expression: snapshot
 ---
 › $
 
-  Google Drive  [App] Connected files
 
-  Press enter to insert or esc to close
+› Google Drive  [App] Connected files
+
+  enter insert · esc close
 
 --- plugin authentication ---
   Plugins

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__worktrees_command_local.snap

@@ -1,8 +1,7 @@
 ---
-source: tui/src/chatwidget/tests/slash_commands.rs
-assertion_line: 3091
+source: tui/src/chatwidget/tests/worktree_picker_tests.rs
 expression: popup
 ---
 › /work
 
-  /worktree  start or continue a conversation in a new worktree
+› /worktree  start or continue a conversation in a new worktree