feat(cli): add ansi_basic theme fallback and offline provider shim for limited‑color terminals

- Detect terminal color support and automatically switch to the new `ansi_basic` theme when only 16‑color support is available.
- Introduce `OfflineProvider` that supplies a placeholder model and friendly messages when no providers are reachable, keeping the TUI usable.
- Add `CONFIG_SCHEMA_VERSION` (`1.1.0`) with schema migration logic and default handling in `Config`.
- Update configuration saving to persist the schema version and ensure defaults.
- Register the `ansi_basic` theme in `theme.rs`.
- Extend `ChatApp` with `set_status_message` to display custom status lines.
- Update documentation (architecture, Vim mode state machine) to reflect new behavior.
- Add async‑trait and futures dependencies required for the offline provider implementation.

crates/owlen-core/src/theme.rs

@@ -209,6 +209,10 @@ pub fn built_in_themes() -> HashMap<String, Theme> {
             "default_light",
             include_str!("../../../themes/default_light.toml"),
         ),
+        (
+            "ansi_basic",
+            include_str!("../../../themes/ansi-basic.toml"),
+        ),
         ("gruvbox", include_str!("../../../themes/gruvbox.toml")),
         ("dracula", include_str!("../../../themes/dracula.toml")),
         ("solarized", include_str!("../../../themes/solarized.toml")),