[tui] Show speed in session header (#13446)

- add a speed row to the startup/session header under the model row
- render the speed row with the same styling pattern as the model row,
using /fast to change
- show only Fast or Standard to users and update the affected snapshots

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/snapshots/codex_tui__app__tests__clear_ui_after_long_transcript_fresh_header_only.snap

@@ -1,6 +1,5 @@
 ---
 source: tui/src/app.rs
-assertion_line: 3452
 expression: rendered
 ---
 ╭─────────────────────────────────────────────╮

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__session_info_availability_nux_tooltip_snapshot.snap

@@ -1,6 +1,5 @@
 ---
 source: tui/src/history_cell.rs
-assertion_line: 2608
 expression: rendered
 ---
 ╭─────────────────────────────────────╮