feat(ui): add file icon resolver with Nerd/ASCII sets, env override, and breadcrumb display

- Introduce `IconMode` in core config (default Auto) and bump schema version to 1.4.0.
- Add `FileIconSet`, `IconDetection`, and `FileIconResolver` to resolve per‑file icons with configurable fallbacks and environment variable `OWLEN_TUI_ICONS`.
- Export resolver types from `owlen-tui::state::file_icons`.
- Extend `ChatApp` with `file_icons` field, initialize it from config, and expose via `file_icons()` accessor.
- Append system status line showing selected icon set and detection source.
- Implement breadcrumb construction (`repo > path > file`) and display in code pane headers.
- Render icons in file tree, handle unsaved file markers, hidden files, and Git decorations with proper styling.
- Add helper `collect_unsaved_relative_paths` and tree line computation for visual guides.
- Provide `Workspace::panes()` iterator for unsaved tracking.
- Update UI imports and tests to cover new breadcrumb feature.

crates/owlen-core/src/config.rs

@@ -14,7 +14,7 @@ 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 = "1.3.0";
+pub const CONFIG_SCHEMA_VERSION: &str = "1.4.0";
 
 /// Core configuration shared by all OWLEN clients
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -724,6 +724,21 @@ pub struct UiSettings {
     pub syntax_highlighting: bool,
     #[serde(default = "UiSettings::default_show_timestamps")]
     pub show_timestamps: bool,
+    #[serde(default = "UiSettings::default_icon_mode")]
+    pub icon_mode: IconMode,
+}
+
+/// Preference for which symbol set to render in the terminal UI.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "lowercase")]
+pub enum IconMode {
+    /// Automatically detect support for Nerd Font glyphs.
+    #[default]
+    Auto,
+    /// Use only ASCII-safe symbols.
+    Ascii,
+    /// Force Nerd Font glyphs regardless of detection heuristics.
+    Nerd,
 }
 
 impl UiSettings {
@@ -771,6 +786,10 @@ impl UiSettings {
         true
     }
 
+    const fn default_icon_mode() -> IconMode {
+        IconMode::Auto
+    }
+
     fn deserialize_role_label_mode<'de, D>(
         deserializer: D,
     ) -> std::result::Result<RoleLabelDisplay, D::Error>
@@ -838,6 +857,7 @@ impl Default for UiSettings {
             show_cursor_outside_insert: Self::default_show_cursor_outside_insert(),
             syntax_highlighting: Self::default_syntax_highlighting(),
             show_timestamps: Self::default_show_timestamps(),
+            icon_mode: Self::default_icon_mode(),
         }
     }
 }