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-tui/src/state/mod.rs

@@ -6,11 +6,13 @@
 //! to test in isolation.
 
 mod command_palette;
+mod file_icons;
 mod file_tree;
 mod search;
 mod workspace;
 
 pub use command_palette::{CommandPalette, ModelPaletteEntry, PaletteGroup, PaletteSuggestion};
+pub use file_icons::{FileIconResolver, FileIconSet, IconDetection};
 pub use file_tree::{
     FileNode, FileTreeState, FilterMode as FileFilterMode, GitDecoration, VisibleFileEntry,
 };