tui: • Working, 100% context dim (#4629)

- add a `•` before the "Working" shimmer
- make the percentage in "X% context left" dim instead of bold

<img width="751" height="480" alt="Screenshot 2025-10-02 at 2 29 57 PM"
src="https://github.com/user-attachments/assets/cf3e771f-ddb3-48f4-babe-1eaf1f0c2959"
/>

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__chat_small_running_h2.snap

@@ -2,5 +2,5 @@
 source: tui/src/chatwidget/tests.rs
 expression: terminal.backend()
 ---
-"  Thinking (0s • esc to interrupt)      "
+"• Thinking (0s • esc to interrupt)      "
 "› Ask Codex to do anything              "

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__chatwidget_exec_and_status_layout_vt100_snapshot.snap

@@ -9,7 +9,7 @@ expression: term.backend().vt100().screen().contents()
   └ Search Change Approved
     Read diff_render.rs
 
-  Investigating rendering code (0s • esc to interrupt)
+• Investigating rendering code (0s • esc to interrupt)
 
 
 › Summarize recent commits

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

@@ -3,7 +3,7 @@ source: tui/src/chatwidget/tests.rs
 expression: terminal.backend()
 ---
 "                                                                                "
-"  Analyzing (0s • esc to interrupt)                                             "
+"• Analyzing (0s • esc to interrupt)                                             "
 "                                                                                "
 "                                                                                "
 "› Ask Codex to do anything                                                      "