Color TUI statusline from active theme (#19631)

## Why

Users have shared that the TUI can feel too visually flat because themes
mostly show up in code syntax highlighting. The configurable statusline
is a natural place to make the active theme more visible, while still
letting users keep the existing monotone statusline if they prefer it.

## What Changed

- Added a statusline styling helper that builds the rendered statusline
from `(StatusLineItem, text)` segments, preserving item identity while
keeping the plain text output unchanged.
- Derived foreground accent colors from the active syntax theme by
looking up TextMate scopes through the existing syntax highlighter, with
conservative ANSI fallbacks when a scope does not provide a foreground.
- Tuned theme-derived colors to keep the accents visible without making
the statusline feel overly bright.
- Added `[tui].status_line_use_colors`, defaulting to `true`, plus a
separated `/statusline` toggle so users can enable or disable
theme-derived statusline colors from the setup UI.
- Updated the live statusline and `/statusline` preview to use the same
styled builder, while keeping terminal-title preview text plain.
- Kept statusline separators and active-agent add-ons subdued while
removing blanket dimming from the whole passive statusline.

## Verification

- `cargo test -p codex-tui status_line`
- `cargo test -p codex-tui theme_picker`
- `cargo test -p codex-tui foreground_style_for_scopes`
- `cargo test -p codex-tui`
- `cargo test -p codex-config`
- `cargo test -p codex-core status_line_use_colors`
- `cargo insta pending-snapshots --manifest-path tui/Cargo.toml`

## Visual

<img width="369" height="23" alt="Screenshot 2026-04-30 at 6 16 08 PM"
src="https://github.com/user-attachments/assets/11d03efb-8e4f-4450-8f4d-00a9659ef4cd"
/>

<img width="385" height="23" alt="Screenshot 2026-04-30 at 6 16 02 PM"
src="https://github.com/user-attachments/assets/a3d89f36-bdc1-42e8-8e84-61350e3999e2"
/>

codex-rs/config/src/types.rs

@@ -634,6 +634,11 @@ pub struct Tui {
     #[serde(default)]
     pub status_line: Option<Vec<String>>,
 
+    /// Color status line items with colors derived from the active syntax theme.
+    /// Defaults to `true`.
+    #[serde(default = "default_true")]
+    pub status_line_use_colors: bool,
+
     /// Ordered list of terminal title item identifiers.
     ///
     /// When set, the TUI renders the selected items into the terminal window/tab title.