Added feature switch to disable animations in TUI (#6870)

This PR adds support for a new feature flag `tui.animations`. By
default, the TUI uses animations in its welcome screen, "working"
spinners, and "shimmer" effects. This animations can interfere with
screen readers, so it's good to provide a way to disable them.

This change is inspired by [a
PR](https://github.com/openai/codex/pull/4014) contributed by @Orinks.
That PR has faltered a bit, but I think the core idea is sound. This
version incorporates feedback from @aibrahim-oai. In particular:
1. It uses a feature flag (`tui.animations`) rather than the unqualified
CLI key `no-animations`. Feature flags are the preferred way to expose
boolean switches. They are also exposed via CLI command switches.
2. It includes more complete documentation.
3. It disables a few animations that the other PR omitted.

codex-rs/tui/src/exec_cell/render.rs

@@ -40,17 +40,21 @@ pub(crate) fn new_active_exec_command(
     parsed: Vec<ParsedCommand>,
     source: ExecCommandSource,
     interaction_input: Option<String>,
+    animations_enabled: bool,
 ) -> ExecCell {
-    ExecCell::new(ExecCall {
-        call_id,
-        command,
-        parsed,
-        output: None,
-        source,
-        start_time: Some(Instant::now()),
-        duration: None,
-        interaction_input,
-    })
+    ExecCell::new(
+        ExecCall {
+            call_id,
+            command,
+            parsed,
+            output: None,
+            source,
+            start_time: Some(Instant::now()),
+            duration: None,
+            interaction_input,
+        },
+        animations_enabled,
+    )
 }
 
 fn format_unified_exec_interaction(command: &[String], input: Option<&str>) -> String {
@@ -168,7 +172,10 @@ pub(crate) fn output_lines(
     }
 }
 
-pub(crate) fn spinner(start_time: Option<Instant>) -> Span<'static> {
+pub(crate) fn spinner(start_time: Option<Instant>, animations_enabled: bool) -> Span<'static> {
+    if !animations_enabled {
+        return "•".dim();
+    }
     let elapsed = start_time.map(|st| st.elapsed()).unwrap_or_default();
     if supports_color::on_cached(supports_color::Stream::Stdout)
         .map(|level| level.has_16m)
@@ -239,7 +246,7 @@ impl ExecCell {
         let mut out: Vec<Line<'static>> = Vec::new();
         out.push(Line::from(vec![
             if self.is_active() {
-                spinner(self.active_start_time())
+                spinner(self.active_start_time(), self.animations_enabled())
             } else {
                 "•".dim()
             },
@@ -347,7 +354,7 @@ impl ExecCell {
         let bullet = match success {
             Some(true) => "•".green().bold(),
             Some(false) => "•".red().bold(),
-            None => spinner(call.start_time),
+            None => spinner(call.start_time, self.animations_enabled()),
         };
         let is_interaction = call.is_unified_exec_interaction();
         let title = if is_interaction {