Modes label below textarea (#9645)

# Summary
- Add a collaboration mode indicator rendered at the bottom-right of the
TUI composer footer.
- Style modes per design (Plan in #D72EE1, Execute matching dim context
style, Pair Programming using the same cyan as text elements).
- Add shared “(shift+tab to cycle)” hint text for all mode labels and
align the indicator with the left footer margin.

NOTE: currently this is hidden if the Collaboration Modes feature flag
is disabled, or in Custom mode. Maybe we should show it in Custom mode
too? I'll leave that out of this PR though

# UI
- Mode indicator appears below the textarea, bottom-right of the footer
line.
- Includes “(shift+tab to cycle)” and keeps right padding aligned to the
left footer indent.

<img width="983" height="200" alt="Screenshot 2026-01-21 at 7 17 54 PM"
src="https://github.com/user-attachments/assets/d1c5e4ed-7d7b-4f6c-9e71-bc3cf6400e0e"
/>

<img width="980" height="200" alt="Screenshot 2026-01-21 at 7 18 53 PM"
src="https://github.com/user-attachments/assets/d22ff0da-a406-4930-85c5-affb2234e84b"
/>

<img width="979" height="201" alt="Screenshot 2026-01-21 at 7 19 12 PM"
src="https://github.com/user-attachments/assets/862cb17f-0495-46fa-9b01-a4a9f29b52d5"
/>

codex-rs/tui/src/bottom_pane/snapshots/codex_tui__bottom_pane__footer__tests__footer_mode_indicator_narrow_overlap_hides.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/bottom_pane/footer.rs
+expression: terminal.backend()
+---
+"  100% context left · ? for shortcuts             "

codex-rs/tui/src/bottom_pane/snapshots/codex_tui__bottom_pane__footer__tests__footer_mode_indicator_wide.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/bottom_pane/footer.rs
+expression: terminal.backend()
+---
+"  100% context left · ? for shortcuts                                                   Plan mode (shift+tab to cycle)  "