feat(accessibility): add high-contrast display modes

Acceptance Criteria:\n- Users can toggle high-contrast and reduced-chrome modes via :accessibility commands.\n- Accessibility flags persist in config and update UI header legends without restart.\n- Reduced chrome removes glass shadows while preserving layout readability.

Test Notes:\n- cargo test -p owlen-tui

crates/owlen-core/src/config.rs

@@ -17,7 +17,7 @@ use std::time::Duration;
 pub const DEFAULT_CONFIG_PATH: &str = "~/.config/owlen/config.toml";
 
 /// Current schema version written to `config.toml`.
-pub const CONFIG_SCHEMA_VERSION: &str = "1.6.0";
+pub const CONFIG_SCHEMA_VERSION: &str = "1.7.0";
 
 /// Provider config key for forcing Ollama provider mode.
 pub const OLLAMA_MODE_KEY: &str = "ollama_mode";
@@ -1814,6 +1814,35 @@ pub struct UiSettings {
     pub keymap_profile: Option<String>,
     #[serde(default)]
     pub keymap_path: Option<String>,
+    #[serde(default)]
+    pub accessibility: AccessibilitySettings,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AccessibilitySettings {
+    #[serde(default = "AccessibilitySettings::default_high_contrast")]
+    pub high_contrast: bool,
+    #[serde(default = "AccessibilitySettings::default_reduced_chrome")]
+    pub reduced_chrome: bool,
+}
+
+impl AccessibilitySettings {
+    const fn default_high_contrast() -> bool {
+        false
+    }
+
+    const fn default_reduced_chrome() -> bool {
+        false
+    }
+}
+
+impl Default for AccessibilitySettings {
+    fn default() -> Self {
+        Self {
+            high_contrast: Self::default_high_contrast(),
+            reduced_chrome: Self::default_reduced_chrome(),
+        }
+    }
 }
 
 /// Preference for which symbol set to render in the terminal UI.
@@ -1957,6 +1986,7 @@ impl Default for UiSettings {
             icon_mode: Self::default_icon_mode(),
             keymap_profile: Self::default_keymap_profile(),
             keymap_path: None,
+            accessibility: AccessibilitySettings::default(),
         }
     }
 }
@@ -2384,6 +2414,13 @@ mod tests {
         assert_eq!(cloud.api_key_env.as_deref(), Some(OLLAMA_API_KEY_ENV));
     }
 
+    #[test]
+    fn default_ui_accessibility_flags_off() {
+        let config = Config::default();
+        assert!(!config.ui.accessibility.high_contrast);
+        assert!(!config.ui.accessibility.reduced_chrome);
+    }
+
     #[test]
     fn ensure_provider_config_aliases_cloud_defaults() {
         let mut config = Config::default();