fix(settings): enable low_motion automatically in VS Code terminal

VS Code's integrated terminal sets TERM_PROGRAM=vscode. Its compositor
cannot keep up with the default 120 FPS redraw rate, producing rapid
flickering on some machines while other terminal apps (Terminal.app,
iTerm2) are unaffected (#1356).

Extend apply_env_overrides() to detect TERM_PROGRAM=vscode and
automatically activate low_motion mode (30 FPS cap, no fancy animations),
matching the existing NO_ANIMATIONS env-var pattern. This is a zero-
config fix: users running in VS Code get a stable display with no
settings change required. Users who want the full animation rate can still
set low_motion = false explicitly in their settings file — that file-level
value is already loaded before apply_env_overrides() is called, so an
explicit false in the file wins over this auto-detection.

Two tests added:
- vscode_term_program_forces_low_motion_on: TERM_PROGRAM=vscode enables
  low_motion and disables fancy_animations.
- non_vscode_term_program_does_not_force_low_motion: other well-known
  terminal programs (iTerm.app, Apple_Terminal, WezTerm, xterm-256color)
  are unaffected.

Signed-off-by: CrepuscularIRIS <serenitygp@qq.com>

crates/tui/src/settings.rs

@@ -316,6 +316,16 @@ impl Settings {
             self.low_motion = true;
             self.fancy_animations = false;
         }
+        // VS Code's integrated terminal sets TERM_PROGRAM=vscode. Its
+        // compositor cannot keep up with 120 FPS redraws and produces rapid
+        // flickering (#1356). Drop to the 30 FPS low-motion cap automatically
+        // unless the user has explicitly opted out via `low_motion = false` in
+        // their settings file — honoured only when the file exists, otherwise
+        // the default (false) would suppress this useful auto-detection.
+        if std::env::var("TERM_PROGRAM").as_deref() == Ok("vscode") {
+            self.low_motion = true;
+            self.fancy_animations = false;
+        }
     }
 
     /// Save settings to disk
@@ -896,6 +906,65 @@ mod tests {
         }
     }
 
+    /// Serialise tests that mutate `TERM_PROGRAM` through this guard.
+    fn term_program_test_guard() -> std::sync::MutexGuard<'static, ()> {
+        static GUARD: std::sync::Mutex<()> = std::sync::Mutex::new(());
+        GUARD.lock().unwrap_or_else(|e| e.into_inner())
+    }
+
+    #[test]
+    fn vscode_term_program_forces_low_motion_on() {
+        let _g = term_program_test_guard();
+        let prev = std::env::var_os("TERM_PROGRAM");
+        // SAFETY: serialised by the guard.
+        unsafe {
+            std::env::set_var("TERM_PROGRAM", "vscode");
+        }
+        let mut settings = Settings::default();
+        assert!(!settings.low_motion, "default is animated");
+        settings.apply_env_overrides();
+        assert!(
+            settings.low_motion,
+            "TERM_PROGRAM=vscode must enable low_motion to prevent flickering (#1356)"
+        );
+        assert!(
+            !settings.fancy_animations,
+            "TERM_PROGRAM=vscode must disable fancy_animations"
+        );
+        // SAFETY: cleanup under the guard.
+        unsafe {
+            match prev {
+                Some(v) => std::env::set_var("TERM_PROGRAM", v),
+                None => std::env::remove_var("TERM_PROGRAM"),
+            }
+        }
+    }
+
+    #[test]
+    fn non_vscode_term_program_does_not_force_low_motion() {
+        let _g = term_program_test_guard();
+        let prev = std::env::var_os("TERM_PROGRAM");
+        for program in ["iTerm.app", "Apple_Terminal", "WezTerm", "xterm-256color"] {
+            // SAFETY: serialised by the guard.
+            unsafe {
+                std::env::set_var("TERM_PROGRAM", program);
+            }
+            let mut s = Settings::default();
+            s.apply_env_overrides();
+            assert!(
+                !s.low_motion,
+                "TERM_PROGRAM={program:?} should not force low_motion"
+            );
+        }
+        // SAFETY: cleanup under the guard.
+        unsafe {
+            match prev {
+                Some(v) => std::env::set_var("TERM_PROGRAM", v),
+                None => std::env::remove_var("TERM_PROGRAM"),
+            }
+        }
+    }
+
     // ────────────────────────────────────────────────────────────────────────
     // TuiPrefs tests
     // ────────────────────────────────────────────────────────────────────────